Circuit Description

PCM monitors the Heated Oxygen Sensor HO2S activity for 100 seconds. During this period, PCM counts the number of Lean/Rich (L/R) and Rich/Lean (R/L) switches. With this information, an average time for all transitions can be determined. PCM then divides R/L average by L/R rich average to obtain a ratio.

Conditions for setting DTC are: no active fuel trim, injector circuit, misfire, AIR, CKP, ECT, EGR, EVAP, IAT, MAF, MAP or TP sensor DTCs set. P0131, P0132 or P0135 not set, system voltage greater than 9 volts, system in closed-loop, engine running for at least 3.3 minutes, ECT greater than 167°F (75°C), engine speed 1000-3000 RPM, MAF 15-40 gm/s and these conditions stable for 3 seconds.

DTC will set when PCM detects degraded (slow) rich-to-lean or lean-to-rich response times for HO2S 1 or HO2S ratio is not within range.