Circuit Description

PCM continuously monitors HO2S activity for 100 seconds. During this period PCM counts the number of times HO2S responds from rich-to-lean and from lean-to-rich and adds the amount of time it took to complete all transitions. With this information, PCM can determine the average time for all transitions. If average time is too slow, DTC will set.

The PCM determines lean-to-rich transitions when HO2S voltage changes from less than 300 mV to greater than 600 mV. The PCM determines rich-to-lean transitions when HO2S voltage changes from greater than 600 mV to less than 300 mV. An HO2S that responds too slowly is most likely defective. Replace HO2S.

For duplication of DTC, ensure:

• DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0200, P0335, P0336, P0351-P0358 or P1258 are not set.
• Fuel system is operating in closed-loop.
• AIR, EGR and catalyst diagnostics are not active.
• Ignition voltage is greater than 9 volts.
• ECT at greater than 122°F (50°C).
• Engine speed between 1000-2000 RPM.
• Engine airflow at 20 gms/s and 50 gms/s.
• EVAP canister purge duty cycle is greater than zero percent.
• Engine run time greater than 120 seconds.