P0138
DTC P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
Circuit Description
To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the Bank 1 HO2S 1 and the Bank 1 HO2S 2 heated oxygen sensors. The Bank 1 HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 1 signal will be far more active than that produced by the Bank 1 HO2S 2 sensor. If the Bank 1 HO2S 2 signal voltage remains excessively high for an extended period of time, Diagnostic Trouble Code P0135 will be set. DTC P0138 is a type A code.
Conditions for Setting the DTC
- No related Diagnostic Trouble Codes.
- Engine is operating in "Closed Loop".
- "Closed Loop" commanded air/fuel ratio is between 14.5 and 14.8.
- Engine coolant temperature is above 60 °C (140 °F).
- Throttle angle is between 3 % and 19 %.
All above conditions met for 3.0 seconds or in "Deceleration Fuel Cutoff' DFCO mode, and one of the following two conditions are met:
- Bank 1 HO2S 2 signal voltage remains above 952 mV during normal "Closed Loop" operation for a total of 106.25 seconds over a 125 - second period of time.
OR
- Bank 1 HO2S 2 signal voltage remains above 474 mV during deceleration fuel cutoff mode operation for 5 seconds.
Action Taken When the DTC Sets
- The PCM will illuminate the malfunction indicator lamp (MIL) the first time the fault is detected.
- The PCM will store conditions which were present when the Diagnostic Trouble Code was set as Freeze Frame and in the Failure Records data.
Conditions for Clearing the MIL/DTC
- The PCM will turn the MIL OFF on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
- A history Diagnostic Trouble Code P0138 will clear after 40 consecutive warm-up cycles have occurred without a fault.
- Diagnostic Trouble Code P0138 can be cleared by using the Scan Tool's "Clear Info" function.
Diagnostic Aids
Check the following items:
- Fuel pressure - An excessively rich fuel mixture can cause a Diagnostic Trouble Code P0138 to be set. Refer to Fuel System Diagnosis.
- Rich injector(s) - Perform "Injector Balance Test."
- Leaking injector - Refer to Fuel System Diagnosis.
- Evaporative emissions (EVAP) canister purge - Check for fuel saturation. If full of fuel, check the canister control and hoses. Refer to Evaporative Emission (EVAP) Control System.
- Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for the presence of fuel.
- TP sensor - An intermittent TP sensor output will cause the system to go rich, due to a false indication of the engine accelerating.
- Shorted Heated Oxygen Sensor (HO2S) - If the HO2S is internally shorted the HO2S voltage displayed on the Tech 2 will be over 1 volt. Silicon contamination of the HO2S can also cause a high HO2S voltage. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is evident, replace the affected HO2S.
- Open HO2S Signal, Low Circuit or Faulty HO2S - A poor connection or open in the HO2S signal or low circuit can cause the DTC to set during deceleration fuel mode. An HO2S which is faulty and not allowing a full voltage swing between the rich and lean thresholds can also cause this condition. Operate the vehicle while monitoring the HO2S voltage with a Tech 2. If the HO2S voltage is limited within a range between 300 mV to 600 mV, check the HO2S signal and low circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the HO2S.
Steps 1 - 4:
Steps 5 - 9:
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Chart:
3. Diagnostic Trouble Code P0138 being set during deceleration fuel mode operation may indicate a condition described in the "Diagnostic Aids" above. If the Diagnostic Trouble Code P0138 test passes while the Failure Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the Diagnostic Trouble Code to be set occurs. This may assist in diagnosing the condition.