Circuit Description

To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. When vehicle is first started, PCM controls fuel delivery in open loop, ignoring HO2S signals and calculating air/fuel ratio based on inputs from engine coolant temperature, throttle position, and mass airflow sensors. PCM will begin using HO2S 1 signal for controlling fuel delivery (Closed Loop), when the following conditions have been met:

• HO2S output signal is varying, indicating that HO2S is hot enough to operate properly.
• ECT sensor indicates coolant temperature is more than 70°F (21°C).
• Time since start-up is at least 15 seconds for a warm engine or 90 seconds for a cold engine.

PCM monitors amount of time for cooling system to achieve closed loop operating temperature. This diagnostic is broken into 3 temperature regions. Diagnostic uses ECT and IAT sensors to determine which region to operate in. The time to achieve closed loop temperature is monitored. A time based decision is made depending on temperature region. Diagnostic will monitor the following conditions prior to setting the DTC:

• Total airflow must exceed a calibrated amount.
• Engine idle time must be less than a calibrated amount.