Circuit Description

PCM supplies a bias voltage (about 450 mV) on HO2S signal high and low circuits. When ignition is turned on, battery voltage is supplied to HO2S heater. As heater reaches operating temperature, HO2S voltage responds by changing from a bias voltage range to normal operation (less than 300 mV). Depending on exhaust gas content, it is possible for HO2S voltage to increase to more than 450 mV.

PCM will run heater test only on a cold start and once during ignition cycle. When engine is started, PCM will determine how much time it takes HO2S voltage to go from above or below bias range threshold. If PCM detects that it took too much time to enter into normal operating range, DTC will set. The time it takes HO2S to reach operating temperature is based on amount of air that flows into engine.

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.
• IAT and ECT are less than 122°F (50°C) and are within 46°F (8°C) of each other at start-up.
• Ignition voltage is 10-18 volts.
• AIR, EGR and catalyst diagnostics are not active.