Decoding Your 1995 Mustang OBD2 Error Codes: A Comprehensive Guide

Understanding the error codes in your 1995 Ford Mustang is crucial for maintaining its performance and longevity. While the 1995 Mustang model year is often considered a transition period in automotive diagnostics, bridging the gap between older EEC-IV systems and the newer OBD2 (On-Board Diagnostics II) standard, knowing how to interpret these codes can save you time and money on repairs. This guide will help you navigate the diagnostic codes you might encounter in your 1995 Mustang, providing insights into potential issues and how to address them.

Whether your Mustang is throwing a check engine light or you’re proactively checking for stored codes, this comprehensive list, adapted from classic Ford EEC-IV systems which share conceptual similarities with early OBD2 implementations, will serve as a valuable resource. Remember, while pinpoint accuracy may require professional diagnostic tools, this guide offers a strong starting point for understanding what your Mustang is trying to tell you.

Understanding Diagnostic Code Types

Before diving into the codes themselves, it’s important to understand the prefixes that often accompany them. These prefixes indicate when the error code was recorded:

• O – Key On, Engine Off code: This code is detected when the ignition is turned on, but the engine is not running. It usually points to issues that can be identified without the engine in operation, such as sensor malfunctions or circuit problems.
• CM – Continuous Memory code: These codes are stored in the vehicle’s computer memory and represent past faults. They can be helpful in identifying intermittent problems or issues that have occurred previously. Clearing these codes and re-testing can help determine if the problem is still present.
• R – Engine Running code: As the name suggests, these codes are detected when the engine is running. They often relate to issues that arise during engine operation, like fuel mixture problems, misfires, or sensor readings that are out of range under load.

Common 1995 Mustang OBD2 & EEC-IV Style Error Codes

While the following codes are rooted in the EEC-IV system prevalent in Mustangs leading up to 1995, they represent fundamental automotive issues that are still relevant in the context of early OBD2 systems and can provide directional diagnostic information for a 1995 Mustang. Always consult a professional diagnostic tool for precise OBD2 codes for your specific vehicle.

• 11 – System Pass (O,CM,R): This is good news! It indicates that the system has passed its self-test and no faults were detected during the specific test condition (Key On Engine Off, Continuous Memory, or Engine Running).

• 12 – RPM Not Within Self Test Lower Limit (R), DC Motor Did Not Move (O,CM,R), Idle Speed Control motor or Air Bypass not controlling idle properly – generally idle too low: This suggests an issue with idle control. The engine RPM might be too low during testing, or the idle speed control system isn’t functioning correctly, potentially leading to a low idle. This could be related to the Idle Air Control (IAC) valve, vacuum leaks, or throttle body issues.

• 13 – RPM Not Within Self Test Upper Limit (R), Idle Speed Control motor or Air Bypass not controlling idle properly – generally idle too high: Similar to code 12, but indicating a high idle issue. The RPM is too high, and the idle control system isn’t effectively reducing it. Again, suspect the IAC valve, but also check for throttle sticking, air leaks, or issues with the throttle position sensor.

• 14 – Profile Ignition Pickup Circuit Failure (CM): This code points to a problem within the ignition system, specifically the Profile Ignition Pickup (PIP) circuit. This circuit is crucial for timing and ignition. Issues here can stem from a faulty PIP sensor (often located within the distributor), wiring problems, or PCM (Powertrain Control Module) malfunction.

• 15 – Readout Memory Test Failed (O), Keep Alive Memory Test Failed (CM): This indicates a failure within the vehicle’s computer memory. “Readout Memory Test Failed” suggests a problem with accessing or reading the computer’s memory during the Key On Engine Off test. “Keep Alive Memory Test Failed” points to issues with the memory that retains stored data (like learned fuel trims and error codes) when the ignition is off. This could indicate a failing PCM or issues with power supply to the PCM.

• 16 – RPM Too Low To Perform o2 Sensor Test (R): This code isn’t necessarily an error itself, but rather an indication that the engine RPM was too low for the system to properly test the oxygen sensors during the Engine Running test. It might suggest an underlying idle problem (see codes 12 and 17).

• 17 – RPM below Self-Test limit with Idle Air Control off (R): This code is similar to 12, further pinpointing a low RPM issue even when the Idle Air Control (IAC) system is commanded off during the test. This strengthens the suspicion of vacuum leaks, mechanical issues affecting idle, or a faulty IAC bypass mechanism.

• 18 – Spark Out (SPOUT) Circuit Open (R), Loss Of Ignition Diagnostic Module Input To PCM/SPOUT Circuit Grounded (CM): “Spark Out Circuit Open” indicates a break in the SPOUT (Spark Output) circuit when the engine is running. This circuit is vital for controlling ignition timing. “Loss Of Ignition Diagnostic Module Input” or “SPOUT Circuit Grounded” points to potential issues with the ignition module or wiring short circuits affecting the SPOUT signal. These codes can lead to misfires and poor engine performance.

• 19 – Failure In PCM Internal Voltage (R), Erratic RPM During Hard Idle Self Test (R), Failure in EEC reference voltage (O): This code signals problems with the PCM itself or its voltage supply. “Failure in PCM Internal Voltage” suggests an internal fault within the computer’s voltage regulation. “Erratic RPM During Hard Idle Self Test” indicates unstable engine speed during the diagnostic test, which could be related to PCM issues or other engine control problems. “Failure in EEC reference voltage” points to a problem with the stable 5-volt reference voltage that the PCM provides to many sensors; without this, sensor readings will be inaccurate.

• 21 – Engine Coolant Temperature (ECT) Sensor Out Of Self Test Range. 0.3 to 3.7 volts(O,R): The Engine Coolant Temperature (ECT) sensor reading is outside the expected range during the self-test. This could be due to a faulty ECT sensor, wiring problems (open or short circuit), or even low coolant levels if the sensor is not properly immersed.

• 22 – Manifold Absolute Pressure/Barometric Pressure Sensor Out Of Self Test Range (O,CM,R): Indicates the Manifold Absolute Pressure (MAP) or Barometric Pressure (BARO) sensor is reading outside its acceptable range during the self-test. This could be a sensor malfunction, vacuum leaks affecting MAP readings, or atmospheric pressure issues impacting BARO readings.

• 23 – Throttle Position Sensor Out Of Self Test Range (O,CM,R): The Throttle Position Sensor (TPS) is reporting a value outside the expected range during the self-test. This is often caused by a faulty TPS, incorrect TPS adjustment, or wiring problems. It can lead to issues with idle, acceleration, and shifting.

• 24 – Intake Air Temperature/Air Charge Temperature Sensor Out Of Self Test Range. 0.3 to 3.7 volts(O,R): Similar to code 21, but for the Intake Air Temperature (IAT) or Air Charge Temperature (ACT) sensor. The sensor reading is out of range, suggesting a faulty IAT/ACT sensor or wiring issues.

• 25 – Knock Not Sensed During Dynamic Response Test (R): During a specific test designed to detect engine knock (detonation), no knock was sensed. While seemingly positive, this code in some contexts can indicate a fault in the knock sensor or its circuit, meaning the system might not be able to detect knock when it actually occurs.

• 26 – Mass Air Flow Sensor was greater than 0.7 volts with engine off (O), MAF sensor was not between 0.2 and 1.5 volts with engine running (R).: Problems with the Mass Air Flow (MAF) sensor readings. “Greater than 0.7 volts with engine off” indicates a possible sensor fault or wiring issue causing a high voltage reading when there should be none. “Not between 0.2 and 1.5 volts with engine running” suggests the MAF sensor is not reporting air flow within the expected range under running conditions, potentially due to a faulty sensor, intake leaks after the MAF, or wiring problems.

• 28 – Loss Of Ignition Diagnostic Module RH Side (CM): Similar to code 18, but specifically indicating a problem with the ignition diagnostic module for the right-hand side (likely referring to a specific bank of cylinders in V-engines, if applicable to the 1995 Mustang configuration).

• 29 – Insufficient Input From Vehicle Speed Sensor -To 1992 (CM), Insufficient Input From Programmable Speedometer/Odometer Module -From 1993 (CM): The vehicle speed sensor (VSS) is not providing a signal, or the signal is insufficient. This can affect speedometer function, cruise control, and potentially transmission operation. Check the VSS sensor itself, its wiring, and connections.

• 31 – EGR Valve Position/Pressure Feedback EGR Circuit Below Minimum Voltage. 0.24 volts (O,CM,R): Exhaust Gas Recirculation (EGR) system fault. Specifically, the EGR valve position or pressure feedback circuit is reporting a voltage below the minimum expected. This points to issues with the EGR valve position sensor (if equipped), the EGR pressure feedback sensor (DPFE sensor), or wiring problems in the EGR control circuit.

• 32 – EGR Valve Position/Pressure Feedback EGR Voltage Below Closed Voltage. 0.24 volts (O,CM,R): Similar to code 31, reinforcing a low voltage issue in the EGR feedback circuit, specifically related to the closed position voltage.

• 33 – EGR Valve Opening Not Detected (CM,R): The system commanded the EGR valve to open, but the opening was not detected. This could be due to a stuck EGR valve (carbon buildup), vacuum leaks in the EGR control system, a faulty EGR solenoid, or issues with the EGR valve position sensor (if present).

• 34 – EGR Valve Position/Pressure Feedback EGR Voltage Above Closed Limit (O,R), PFE or EVP circuit has intermittently failed above the closed limit of 0.67 volts (CM): Opposite of codes 31 and 32, indicating a high voltage issue in the EGR feedback circuit, specifically above the closed limit. This could be a faulty EGR position/pressure sensor, wiring shorts, or a PCM issue.

• 35 – EGR Valve Position/EGR Pressure Feedback EGR Circuit Above Maximum Voltage of 4.81 volts (O,R), PFE or EVP circuit has intermittently failed above the maximum limit of 4.81 volts (CM): Further reinforces a high voltage issue in the EGR feedback circuit, now exceeding the maximum limit.

• 36 – System Indicates Lean At Idle (R): The system is detecting a lean air/fuel mixture at idle. This means there’s too much air or not enough fuel. Possible causes include vacuum leaks, lean fuel injectors, low fuel pressure, or a faulty oxygen sensor falsely indicating lean conditions.

• 37 – System Indicates Rich At Idle (R): Conversely, the system detects a rich air/fuel mixture at idle – too much fuel or not enough air. Potential causes include leaky fuel injectors, high fuel pressure, a faulty oxygen sensor falsely indicating rich conditions, or restricted air intake.

• 39 – AXOD converter bypass clutch not applying properly (CM): (If applicable to 1995 Mustang with AXOD transmission) This code points to a problem with the torque converter clutch (TCC) in the AXOD automatic transmission. The TCC is not engaging or applying correctly. This could be due to issues with the TCC solenoid, hydraulic problems within the transmission, or PCM control issues.

• 41 – System Indicates Lean -passenger side(R), No o2 Sensor Switching Detected. always lean -passenger side(CM): Lean condition specifically on the passenger side (bank 1 if V-engine). “No o2 Sensor Switching Detected” suggests the oxygen sensor on that side is not cycling between rich and lean, but stuck in a lean reading. This could be a faulty oxygen sensor, exhaust leaks before the sensor, intake leaks affecting that bank, or fuel delivery issues to those cylinders.

• 42 – System Indicates Rich -passenger side(R), No o2 Sensor Switching Detected. always rich -passenger side(CM): Rich condition on the passenger side (bank 1). Oxygen sensor stuck in a rich reading. Similar causes to code 42, but leaning towards issues like faulty injectors on that bank, high fuel pressure, or exhaust restrictions.

• 43 – o2 Sensors indicate lean at Full Throttle (CM): Lean condition detected by the oxygen sensors when the engine is at full throttle. This is often indicative of fuel delivery problems – the engine isn’t getting enough fuel under high demand. Check fuel pump, fuel filter, fuel pressure regulator, and fuel injectors.

• 44 – Secondary Air System Inoperative. bank one, passenger side (R): The secondary air injection system is not working correctly on the passenger side (bank 1). This system is designed to reduce emissions during cold starts. Problems could be with the secondary air pump, diverter valve, bypass valve, or associated plumbing.

• 45 – Secondary Air Upstream During Self Test (R): Secondary air is being detected upstream of where it should be during the self-test. This could point to incorrect plumbing in the secondary air system or valve malfunctions.

• 46 – Secondary Air Not Bypassed During Self Test (R): The secondary air system is not bypassing as expected during the self-test. This could be a problem with the bypass valve or its control system.

• 47 – Measured Air Flow Low At Base Idle (R): The Mass Air Flow (MAF) sensor is reporting lower than expected air flow at base idle. This can indicate vacuum leaks, a faulty MAF sensor underreporting air flow, or restrictions in the air intake system.

• 48 – Measured air flow too high at base idle (R): Conversely, the MAF sensor is reporting higher than expected air flow at base idle. This is less common, but could be due to a faulty MAF sensor overreporting, or less likely, an issue causing excessive air intake at idle.

• 49 – 1–2 Shift Error (CM): (If applicable to automatic transmission) A problem with the 1-2 shift in the automatic transmission. Could be related to solenoids, hydraulic issues, or mechanical problems within the transmission.

• 51 – Engine Coolant Temperature Circuit Open (CM,O): Engine Coolant Temperature (ECT) sensor circuit is open. This means there’s a break in the wiring to the ECT sensor, or the sensor itself is disconnected or faulty in an “open circuit” manner.

• 52 – Power Steering Pressure Switch Circuit Open (O), Power Steering Pressure Switch Circuit Did Not Change States (R): Problems with the Power Steering Pressure (PSP) switch. “Circuit Open” indicates a break in the wiring. “Did Not Change States” suggests the switch is not registering a change in pressure when the steering is turned, which it should do. This can affect idle speed control and potentially other systems that use PSP switch input.

• 53 – Throttle Position Sensor Circuit Above Maximum Voltage (CM,O): Throttle Position Sensor (TPS) circuit voltage is too high. This could be a short to voltage in the TPS wiring, or a faulty TPS sensor reporting an excessively high voltage.

• 54 – Intake Air Temperature/Air Charge Temperature Circuit Open (CM,O,R): Intake Air Temperature (IAT) or Air Charge Temperature (ACT) sensor circuit is open, similar to code 51 but for the IAT/ACT sensor.

• 55 – Key Power Check (R): A general system check, sometimes indicating a successful system pass in some contexts, but in others can point to a power supply issue to the PCM during the Engine Running test. Context is important.

• 56 – Mass Air Flow/Vane Air Flow Sensor Circuit Above Maximum Voltage (O,CM,R), MAF Sensor short to power (CM,R), Transmission Fluid Temperature Circuit Open (CM,O): Multiple possibilities: MAF/VAF sensor circuit voltage too high (short to voltage or faulty sensor); or a short to power specifically in the MAF sensor circuit; or (if applicable) Transmission Fluid Temperature (TFT) sensor circuit is open.

• 57 – AXOD Neutral pressure switch circuit failed open (CM): (If applicable to AXOD transmission) Neutral pressure switch circuit in the AXOD transmission is open. This switch is used to detect when the transmission is in neutral or park.

• 58 – Idle Tracking Switch Circuit Open Or Grounded -CFI (O,R), Vane Air Temperature Sensor Input Greater Than Self Test Maximum -EFI (CM,O): For CFI (Central Fuel Injection) systems: Idle Tracking Switch circuit issue (open or grounded). For EFI (Electronic Fuel Injection) systems: Vane Air Temperature sensor input too high.

• 59 – 2–3 Shift Error (CM), AXOD 4/3 Pressure Switch Circuit Failed Closed (O), AXOD 4/3 Pressure Switch Circuit Failed Open (C): (If applicable to automatic transmission) 2-3 shift error; or AXOD 4/3 pressure switch circuit issue (failed closed or failed open).

• 61 – ECT Sensor Circuit Grounded (O,CM,R): Engine Coolant Temperature (ECT) sensor circuit is grounded (shorted to ground).

• 62 – Torque Converter Clutch Error (C,M), AXOD 4/3 Or 3/2 Pressure Switch Circuit Grounded (O): (If applicable to automatic transmission) Torque Converter Clutch (TCC) error; or AXOD 4/3 or 3/2 pressure switch circuit grounded.

• 63 – Throttle Position Sensor Circuit Below Minimum Voltage (O,CM,R): Throttle Position Sensor (TPS) circuit voltage too low (short to ground or faulty sensor).

• 64 – Intake Air Temperature/Air Charge Temperature Circuit Grounded (O,CM,R): Intake Air Temperature (IAT) or Air Charge Temperature (ACT) sensor circuit grounded.

• 65 – Transmission Control Switch/Overdrive Cancel Switch Circuit Did Not Change States (R): (If applicable to automatic transmission) Transmission Control Switch or Overdrive Cancel Switch circuit did not change states – the system didn’t detect a change in switch position when expected.

• 66 – , MAF signal below minimum test voltage of .4vdc (O,CM,R), Transmission Fluid Temperature Circuit Grounded (CM,O): MAF signal too low (short to ground or faulty sensor); or (if applicable) Transmission Fluid Temperature (TFT) sensor circuit grounded.

• 67 – Park Neutral Position Switch Circuit Open; A/C On -Manual (O), Manual Lever Position Sensor Out Of Range/A/C On (CM,O), Clutch Switch Circuit Failure (CM): Park/Neutral Position (PNP) switch circuit open (manual transmission, A/C on); or Manual Lever Position (MLP) sensor out of range (A/C on); or Clutch Switch circuit failure.

• 68 – Transmission recently overheated, or TOT sensor intermittently failed below 0.2 volts (CM): (If applicable to automatic transmission) Transmission Over Temperature (TOT) condition detected, or TOT sensor intermittently failed low.

• 69 – 3–4 Shift Error (CM): (If applicable to automatic transmission) 3-4 shift error.

• 71 – Software re-initialization detected or Cluster Control Assembly circuit failed (CM): PCM software re-initialization detected (possibly due to power interruption or PCM issue); or Cluster Control Assembly circuit failure (instrument cluster related).

• 72 – Insufficient Manifold Absolute Pressure/Mass Air Flow Change During Dynamic Response Test (R): During a dynamic response test (like throttle snap test), insufficient change in MAP or MAF sensor readings was detected. This can indicate slow sensor response, vacuum leaks, or intake restrictions.

• 73 – Insufficient Throttle Position Change During Dynamic Response Test (O,R): Insufficient change in Throttle Position Sensor (TPS) reading during a dynamic response test. Could be a faulty TPS or throttle linkage issues.

• 74 – Brake On/Off Circuit Open–Not During Self Test (CM,R): Brake On/Off (BOO) switch circuit open – detected outside of the self-test. This switch is used for various systems like cruise control and torque converter clutch.

• 75 – Brake On/Off Circuit Closed/ECA Input Open (R): Brake On/Off (BOO) switch circuit closed when it shouldn’t be, or ECA (Electronic Control Assembly/PCM) input from the BOO switch is open.

• 76 – Insufficient Vane Air Flow Sensor Output Change During Dynamic Response Test (R): Insufficient change in Vane Air Flow (VAF) sensor output during a dynamic response test (similar to code 72 for MAF/MAP).

• 77 – Operator Error Dynamic Response Test (R): Operator error during the dynamic response test – the test procedure wasn’t followed correctly.

• 78 – Re-Initialization Check (R): System re-initialization check – similar to code 71, possibly indicating a PCM reset or power interruption.

• 79 – A/C On/Defrost On During Self Test (O): Air Conditioning or Defrost was on during the Key On Engine Off self-test. These should be off for accurate testing.

• 81 – Secondary Air Diverter Circuit Failure (O): Secondary Air Diverter valve circuit failure.

• 82 – Secondary Air Bypass Circuit Failure (O): Secondary Air Bypass valve circuit failure.

• 83 – EGR Control solenoid circuit failure (O): EGR Control solenoid circuit failure.

• 84 – EGR Vacuum Regulator (EVR) Control Circuit Failure (O): EGR Vacuum Regulator (EVR) control circuit failure.

• 85 – CANP Circuit Failure (O): CANP (Canister Purge) circuit failure – related to the evaporative emissions system.

• 86 – Adaptive fuel limit reached (CM), 3 – 4 Shift solenoid circuit failure (O): Adaptive fuel learning has reached its limit (the PCM has made maximum adjustments to fuel trims); or 3-4 shift solenoid circuit failure (automatic transmission).

• 87 – Primary Fuel Pump Circuit Failure (CM,O): Primary Fuel Pump circuit failure.

• 88 – Loss Of Dual Plug Input Control (CM), Throttle Kicker Fault (?) : Loss of dual plug input control (if applicable to engine type); or Throttle Kicker fault (for systems equipped with a throttle kicker solenoid for idle control).

• 89 – Converter clutch solenoid circuit failure (CM): (If applicable to automatic transmission) Torque Converter Clutch (TCC) solenoid circuit failure.

• 91 – o2 Sensor Circuit Indicates Lean -driver side(R), No o2 Sensor Switching Detected. always lean -driver side(CM), Shift Solenoid 1 Circuit Failure (O): Lean condition on the driver’s side (bank 2 if V-engine), oxygen sensor stuck lean on that side; or Shift Solenoid 1 circuit failure (automatic transmission).

• 92 – o2 Sensor Circuit Indicates Rich -driver side(R), No o2 Sensor Switching Detected. always rich -driver side(CM), Shift Solenoid 2 Circuit Failure (O): Rich condition on the driver’s side (bank 2), oxygen sensor stuck rich; or Shift Solenoid 2 circuit failure.

• 93 – Converter Clutch Solenoid Circuit Failure (O): Torque Converter Clutch (TCC) solenoid circuit failure (same as code 89).

• 94 – Secondary Air Injection Inoperative. bank two, driver side (R), Torque Converter Clutch Solenoid Circuit Failure (O): Secondary air injection inoperative on the driver’s side (bank 2); or Torque Converter Clutch solenoid circuit failure.

• 95 – Fuel Pump Circuit Open-PCM To Motor Ground (CM,O): Fuel Pump circuit is open between the PCM and the fuel pump motor ground. This indicates a problem in the fuel pump wiring, ground connection, or potentially the PCM’s fuel pump relay control. This is a critical code as it directly relates to fuel delivery.

• 96 – Fuel Pump Circuit Open-Battery To PCM (CM,O): Fuel Pump circuit is open between the battery and the PCM. This indicates a power supply problem to the fuel pump relay or PCM fuel pump control circuit.

• 97 – Transmission Control Indicator Lamp Circuit Failure (O): (If equipped) Transmission Control Indicator Lamp circuit failure – problem with the “O/D Off” light or similar transmission warning light circuit.

• 98 – Hard Fault Present (R): A general “hard fault” is present during the Engine Running test. This often indicates a significant, currently active fault that needs further diagnosis. It may be accompanied by other more specific codes.

• 99 – Electronic Pressure Control Circuit Failure (CM,O): (If applicable to automatic transmission) Electronic Pressure Control (EPC) circuit failure. This circuit controls transmission line pressure electronically.

Cylinder Balance Test Codes

These codes are generated during a specific cylinder balance test, which evaluates the contribution of each cylinder to engine performance.

• 10 – Cylinder #1 Failed Cylinder Balance Test
• 20 – Cylinder #2 Failed Cylinder Balance Test
• 30 – Cylinder #3 Failed Cylinder Balance Test
• 40 – Cylinder #4 Failed Cylinder Balance Test
• 50 – Cylinder #5 Failed Cylinder Balance Test
• 60 – Cylinder #6 Failed Cylinder Balance Test
• 70 – Cylinder #7 Failed Cylinder Balance Test
• 80 – Cylinder #8 Failed Cylinder Balance Test
• 90 – All cylinders passed Cylinder Balance Test

These codes indicate which cylinder(s) are not contributing effectively during the cylinder balance test. A failed cylinder balance test can be caused by issues like:

• Misfires: Spark plug, ignition wire, distributor, or ignition coil problems.
• Fuel Injector Issues: Clogged or malfunctioning fuel injectors.
• Compression Problems: Worn piston rings, valve issues, or head gasket leaks.

Conclusion

Diagnosing your 1995 Mustang using these codes is a valuable first step in troubleshooting potential problems. While this guide provides a comprehensive overview, remember that accurate diagnosis often requires professional tools and expertise. If you are unsure about interpreting these codes or performing repairs, it is always recommended to consult a qualified mechanic. Understanding these potential error codes empowers you to communicate effectively with your mechanic and make informed decisions about your Mustang’s care and maintenance, ensuring it continues to run smoothly for years to come.