Encountering difficulties when trying to communicate with your car’s Powertrain Control Module (PCM) via the OBD2 port can be a frustrating roadblock in automotive diagnostics. This issue prevents you from reading trouble codes, monitoring live data, and performing essential maintenance tasks. Let’s explore a systematic approach to diagnose why it might be Difficult To Talk To Pcm Through Obd2.
First, it’s crucial to check the basics. A simple yet often overlooked component is Fuse #22 in the under-dash fuse panel (location may vary by vehicle, always consult your owner’s manual). While you might have checked this fuse before, it’s worth replacing it with a new one and verifying voltage on both sides of the fuse socket with a multimeter. Ideally, you should be seeing battery voltage, approximately 12V, here. If you’re seeing significantly lower voltage, like 2.0V as in some cases, this is a red flag and indicates a potential issue in the power supply to the OBD2 system or PCM.
Sometimes, a faulty module on the Controller Area Network (CAN) bus can disrupt communication. The Instrument Cluster (INST) is a known culprit. Try disconnecting the instrument cluster by unplugging its two connectors. Additionally, if your vehicle has an aftermarket Trailer Brake Controller (TBC), disconnect it as well. These components can sometimes interfere with the CAN bus and prevent OBD2 communication. Disconnecting them temporarily isolates them as potential causes.
Voltage issues are paramount when diagnosing OBD2 communication problems. At the EGR valve connector (even if the EGR is deleted, the connector might still be present), you should expect to find 12V power on the red wire and 5V on the Brown/White (BN/WH) voltage reference wire. These voltages are supplied by the PCM, and their absence or deviation indicates a problem with the PCM’s power supply or internal circuits. Always probe these voltages with the connector disconnected from the EGR valve to avoid false readings.
Another critical voltage check is at Pin #16 of the Data Link Connector (DLC), your OBD2 port. Pin #16 should consistently have battery voltage, around 12V, at all times. This pin provides power to the OBD2 scan tool. If there’s no voltage or significantly low voltage here, the scan tool won’t power up or communicate correctly.
If basic checks and component disconnections haven’t resolved the communication issue, try disconnecting the PCM connectors themselves. Specifically, disconnect the center PCM connector and the connector closest to the radiator. This step helps rule out issues originating from the PCM or its immediate connections.
Furthermore, certain sensors and actuators can, in rare cases, cause communication problems when they fail in a specific way. Consider disconnecting the following sensors: MAP sensor (Manifold Absolute Pressure), MAF sensor (Mass Air Flow), WIF sensor (Water-in-Fuel), and the fuel pump. While sensors like the APP sensor (Accelerator Pedal Position), brake pedal switch, Barometric pressure sensor, VSS (Vehicle Speed Sensor), and IAT1 sensor (Intake Air Temperature 1) are less likely to cause such issues, focusing on the more common culprits first is a practical approach.
Finally, consider the Throttle Position Sensor (TPS). A recurring P0121 code, related to the TPS, prior to the loss of OBD2 communication could be significant. A fault in the TPS circuit might indeed contribute to a “no communication” scenario. While less common, it’s a factor to keep in mind, especially if there’s a history of TPS-related trouble codes.
In conclusion, diagnosing why it’s difficult to talk to the PCM through OBD2 requires a systematic approach. Start with the simple checks like fuses and voltage at key points. Progress to disconnecting potentially interfering modules and sensors. By methodically eliminating potential causes, you can narrow down the source of the communication breakdown and take appropriate steps for repair.
