For 1990-2009 cars
Engine Controls Components
Cell 21: EVAP and EGR Controls
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### 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 PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The 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 HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the HO2S 2 signal voltage remains excessively low for an extended period of time, DTC P0137 will be set.

### Conditions for Setting the DTC

• | No active misfire, fuel injector circuit, TP sensor, EVAP, IAT sensor, MAP sensor, Fuel trim, EGR, ECT sensor, MAF sensor or CKP sensor DTCs present.

• | Closed loop commanded air/fuel ratio is between 14.5 and 14.8.

• | Throttle angle is between 3% and 40%.

• | Ignition voltage is more than 9.0 volts.

• | Engine coolant temperature greater than 75°C (167°F).

• | HO2S 2 signal voltage remains below 10 mV during normal closed loop operation for 224 seconds.

OR

HO2S 2 signal voltage remains below 600 mV during power enrichment mode fuel control operation for 360 seconds.

### Action Taken When the DTC Sets

• | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.

• | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.

• | The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• | The DTC can be cleared by using the scan tool.

### Diagnostic Aids

Check for the following conditions:

• | Heated oxygen sensor wiring. The sensor pigtail may be mispositioned and contacting the exhaust system. Refer to Wiring Repairs .

• | Poor PCM to engine grounds. Refer to Power and Grounding Component Views .

• | Fuel pressure. A condition which causes a lean exhaust can cause DTC P0137 to set. The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0137 may be set. Refer to Fuel System Diagnosis .

• | Lean injector(s). Perform Fuel Injector Balance Test .

• | Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and Crankcase Ventilation system.

• | Exhaust leaks. An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the DTC P0137 to set. Check for exhaust leaks near the HO2S 2 sensor.

• | Fuel contamination. Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

### Test Description

The numbers below refer to the step numbers on the diagnostic table:

1. DTC P0137 being set in power enrichment mode conditions may indicate a condition described in the Diagnostic Aids above. If the DTC P0137 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

1. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed. Refer to PCM Replacement and Programming Procedures in Powertrain Control Module (PCM) and Sensors.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain OBD System Check performed? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Engine at operating temperature.
2. Operate vehicle within parameters specified under Conditions for Setting the DTC criteria included in Diagnostic Support.
3. Monitor HO2S 2 voltage displayed on the scan tool HO2S data list.

Does the HO2S 2 voltage remain below the specified value?

200 mV

Go to Step 4

Go to Step 3

3 |

1. Turn the ignition on, and leave the engine OFF.
2. Review and record scan tool Fail Records data and note parameters.
3. Operate vehicle within Fail Records conditions as noted.
4. Using a scan tool, monitor Specific DTC info for DTC P0137 until the DTC P0137 test runs.
5. Note test result.

Does scan tool indicate DTC P0137 failed this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4 | Disconnect HO2S 2 and jumper the HO2S 2 low circuit (PCM side) to ground. Does scan tool indicate HO2S 2 voltage near the specified value? | 450 mV | Go to Diagnostic Aids | Go to Step 5
5 |

1. Turn OFF the ignition.
2. Disconnect the PCM, and check the HO2S 2 signal circuit for a short to ground or a short to the sensor ground circuit.

Is the HO2S 2 signal circuit shorted?

--

Go to Step 6

Go to Step 7

6 | Repair the HO2S 2 signal circuit. Refer to Wiring Repairs in Wiring Systems . Is the action complete? | -- | Go to Step 8 | --
7 | Important: Replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 8 | --
8 |

1. Turn the ignition ON, with the engine OFF.
2. Clear DTCs.
3. Review and record scan tool Fail Records data.
4. Operate vehicle within Fail Records conditions as noted.
5. Using a scan tool, monitor Specific DTC info for DTC P0137 until the DTC P0137 test runs.
6. Note test result.

Does scan tool indicate DTC P0137 failed this ignition?

--

Go to Step 2

System OK