Run Mode

WARNING: This page is about a different car, the 2005 Buick LeSabre. However, it is still accessible from the selected car via links, so may be relevant.

The run mode has 2 conditions called open loop and closed loop. When the engine is first started and the engine speed is above a predetermined RPM, the system begins Open Loop operation. The PCM ignores the signal from the heated oxygen sensor (HO2S) and calculates the air/fuel ratio based on inputs from the ECT, MAF, MAP, and TP sensors. The system stays in open loop until meeting the following conditions:

Both HO2S have varying voltage output, showing that they are hot enough to operate properly. This depends upon the engine temperature.
The ECT sensor is above a specified temperature.
A specific amount of time has elapsed after starting the engine.

Specific values for the above conditions exist for each different engine, and are stored in the electrically erasable programmable read-only memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio (injector on-time) based upon the signal from various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.