P0135

CIRCUIT DESCRIPTION
Heating elements inside the heated oxygen sensor (HO2S) minimize the time required for the sensors to reach operating temperature, and to provide an accurate voltage signal. A low side driver within the engine control module (ECM) is pulse width controlled to provide current to the heater elements. During warm-up the ECM will pulse the heaters ON-OFF to prevent thermal shock to the sensor components from moisture in the exhaust system. The ECM will not allow continuous HO2S heating until calibrated limits of time, temperature, and intake airflow have been reached. The ECM continuously monitors the HO2S heater current draw and operating state by briefly turning OFF the heater low side driver at regular intervals. A small reference voltage is present at the heater low control circuit. When the low side driver is commanded ON, the reference voltage is low, 2.6-4.6 volts. When the low side driver is commanded OFF, the reference voltage is high, close to battery voltage. The ECM calculates the HO2S heater element resistance based on the actual heater current. If the ECM detects that the HO2S heater element resistance is too high, DTCs P0135, P0141, P0155, or P0161 will set.

Each HO2S has the following circuits:
- HO2S high signal
- HO2S low signal
- HO2S heater ignition 1 voltage
- HO2S heater low control

CONDITIONS FOR RUNNING THE DTC
- DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0157, P0158, P0160, or P0161 are not set.
- The engine is running.
- The calc converter temp parameter is between 330-600°C (626-1,112°F).
- The battery voltage is between 10.5-16 volts.

CONDITIONS FOR SETTING THE DTC
The ECM detects that the affected HO2S Heater Current is not within the expected range, when the HO2S heater is commanded ON.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

TEST DESCRIPTION

Steps 1-2:

Steps 3-6:

Steps 7-12:

The number below refer to the step number on the diagnostic table.
4. This test must not be performed until the HO2S heater has cooled and stabilized for at least 15 minutes. Heater resistance is typically about 10 ohms at room temperature.