P0135

Circuit Description
This diagnostic test is used to determine if the heating element in the rear oxygen sensor is operating properly. An oxygen sensor acts as a battery because it creates its own voltage once it reaches operating temperature. This voltage is produced when the oxygen content in the exhaust stream is different than that in the atmosphere. The heating element is energized through CKT 639 when the ignition is ON and allows the oxygen sensor to reach operating temperature quickly. This in turn allows the PCM to enter Closed Loop fuel control quicker. When cold the voltage that the PCM measures on the 02 sensor HI and LOW inputs is called bias voltage. If it takes too long for the 02 sensor to very above or below the bias voltage within a specific time since key ON DTC P0135 will set.

Conditions for Running the DTC
^ DTCs P0101, P0102, P0103, P0117, P0118, and P0134 not set.
^ Engine coolant temperature dropped at least 10°C (50°F) from the end of the last ignition cycle to the beginning of this ignition cycle.
^ Ignition voltage 11 volts or more.
^ Average rear heated oxygen sensor bias voltage between 352 mv and 546 mv.
^ Engine did not stall during test.

Conditions for Setting the DTC
The PCM monitors the rear heated oxygen sensor input to determine when its voltage becomes either 151 millivolt less than or 151 millivolt greater than average rear heated oxygen sensor bias voltage for 0.1 second. If it takes too long, DTC P0135 will set.

Action Taken When the DTC Sets
^ The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
^ A Last Test Failed DTC will clear when the diagnostic runs and does not fail.
^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any diagnostic test.
^ Use a scan tool to clear DTCs.
^ Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Conditions for Clearing the MIL/DTC
^ The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
^ A Last Test Failed DTC will clear when the diagnostic runs and does not fail.
^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any diagnostic test.
^ Use a scan tool to clear DTCs.
^ Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Clearing Diagnostic Trouble Codes in Powertrain Control Module Description.

Diagnostic Aids

Notice: Refer to HO2S wire repair notice in Oxygen Sensor. Service and Repair

An intermittent open in CKT 639 or 1350 may cause DTC P0135 to set. Additionally, a malfunctioning rear oxygen sensor heater may be the cause.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
3. When starting with a cool sensor, amperage should start at 1.5 amps and slowly drop to about 0.9 amps. This verifies that the heater is actually producing heat.