Circuit Description

PCM monitors Heated Oxygen Sensor (HO2S) activity for 100 seconds. During this period, PCM counts number of times that HO2S switches from rich to lean and lean to rich and adds amount of time it took to complete all transitions.

With this information, an average time for all transitions can be determined. PCM then divides rich to lean average by lean to rich average to obtain a ratio. If HO2S ratio is not within range, DTC will set.

Conditions for setting DTC:

• DTCs P0102, P0103, P0112, P0113, P0117, P0118, P0125, P0335, P0336, P0351-P0358, P1120, P1220, P1221 or P1258 not set.
• System operating in closed loop.
• Engine speed 1000-2300 RPM.
• Engine operating longer than 2 seconds.
• MAF 15-35 gm/s.
• Ignition voltage greater than 9 volts.
• Engine coolant temperature greater than 149°F (65°C).
• EVAP purge duty cycle greater than zero percent.
• PCM determines that HO2S transition time ratio is not at expected value.