Run Mode

The run mode has 2 conditions called Open Loop and Closed Loop. When the engine is first started the system is in "Open Loop" operation. In Open Loop, the ECM ignores the signal from the Heated Oxygen Sensor (HO2S). It calculates the air/fuel ratio based on inputs from the TP, ECT, and MAF sensors. The system remains in Open Loop until the following conditions are met:

• The HO2S has a varying voltage output showing that it is hot enough to operate properly (this depends on temperature).
• The ECT has reached a specified temperature.
• A specified amount of time has elapsed since starting the engine.
• Engine speed has been greater than a specified RPM since start-up.

The specific values for the above conditions vary with different engines and are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the system enters Closed Loop operation. In Closed Loop, the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.