Many drivers and car enthusiasts often encounter terms like “open loop” and “closed loop” when discussing engine performance and emissions, especially in the context of OBD2 diagnostics. If your mechanic mentioned your car not entering “closed loop” and failing emissions because OBD readiness monitors for the fuel system, catalyst, and O2 sensors are “not ready,” understanding what this means is crucial. This article will clarify the difference between open loop and closed loop systems and explain why closed loop operation is essential for OBD2 readiness.
In essence, your car’s engine control system operates in two primary modes: open loop and closed loop. Open loop operation typically occurs when the engine is first started, particularly when cold. During this phase, the Powertrain Control Module (PCM), or engine computer, disregards the readings from the oxygen sensors in the exhaust system. This is because oxygen sensors require heat to function correctly and provide accurate data. Instead, the PCM relies on pre-programmed maps and inputs from other sensors like the Manifold Absolute Pressure (MAP) sensor, coolant temperature sensor, and intake air temperature sensor to estimate the ideal air-fuel mixture. The goal in open loop is to get the engine running smoothly and warm up the catalytic converter and oxygen sensors quickly.
Once the oxygen sensors reach their optimal operating temperature and the engine coolant temperature reaches a certain threshold, the engine management system transitions into closed loop operation. In closed loop, the PCM actively uses the oxygen sensors as feedback to precisely control the air-fuel ratio (AFR). Oxygen sensors detect the amount of oxygen in the exhaust gas, which directly indicates whether the fuel mixture is lean (too much oxygen) or rich (too little oxygen). This feedback loop allows the PCM to make real-time adjustments to the amount of fuel injected into the engine, striving to maintain the ideal stoichiometric AFR (approximately 14.7:1 for gasoline engines). This precise control is critical for optimal engine efficiency, performance, and minimizing emissions.
For your vehicle to pass an emissions inspection and for the OBD readiness monitors to set to “ready,” the PCM must enter closed loop operation. The readiness monitors are self-tests that the OBD2 system performs to ensure all emissions control components are functioning correctly. Monitors like the fuel system monitor, catalyst monitor, and oxygen sensor monitor specifically require the system to be in closed loop to accurately assess their performance. If your car is not entering closed loop, these monitors will not complete their tests and will remain in a “not ready” state, leading to inspection failure. Addressing the issue preventing your car from entering closed loop is therefore necessary to resolve your OBD readiness issues and pass your emissions test.
