Circuit Description

HO2S-1 produces a voltage that varies between .1 and .9 volt under normal operating conditions. PCM monitors this voltage and determines if exhaust is lean or rich. If voltage input at PCM is about .1 volt, exhaust is lean. If voltage input at PCM is about .9 volt, exhaust is rich. PCM constantly monitors HO2S-1 signal during closed loop operation and compensates for a lean or rich condition by adjusting injector pulse width as necessary.

DTC P0125 will set when engine is at normal operating temperature, and HO2S-1 output does not indicate RICH with engine speed at 1500 RPM or more, vehicle speed at 25-62 MPH, throttle valve position is open, and engine has been running for at least 140 seconds.

When DTC sets, MIL will illuminate after 2 consecutive ignition cycles in which diagnostic runs with active fault. PCM will record operating conditions at time of failure and store information in freeze frame buffer.