Circuit Description
The VCM supplies a voltage of approximately 0.45 volt (450 mV) between the HO2S High and the HO2S Low circuits.
Important: If the voltage is measured with a 10 megohm digital voltmeter, the voltage may read as low as 0.32 volt. The Heated Oxygen Sensor (HO2S) varies the voltage from approximately 1.0 volt (1000 mV) during rich conditions to 0.10 volt (100 mV) during lean conditions.
The Sensor acts like an open circuit and produces no voltage at temperatures below 360°C (600°F). The DTC P0137, P0138 or an open sensor circuit or the cold sensor causes the Open Loop operation. The DTC P0138 determines if the HO2S or the circuit is shorted to high by checking for a rich condition during a steady throttle. This DTC is a type B DTC.
Conditions for Setting the DTC
| • | No TP sensor DTCs. |
| • | No EVAP DTCs. |
| • | No IAT sensor DTCs. |
| • | No MAP sensor DTCs. |
| • | No ECT sensor DTCs. |
| • | No MAF sensor DTCs. |
| • | No intrusive test in progress. |
| • | No device controls active. |
| • | The system voltage measures at least 9.0 volts. |
Test Enable (Rich Test)
| • | Closed Loop. |
| • | Air to fuel ratio is at least 14.5 but less than 14.8. |
| • | The TP is greater than 0% but less than 50%. |
| • | The above must meet for 5 seconds. |
| • | No related HO2S DTCs since code clear. |
| • | The HO2S voltage measures greater than 0.976 volts (976 mV). |
Test Enable (DFCO Rich Test)
| • | Decel Fuel cutoff mode is active. |
| • | Closed Loop. |
| • | The time elapsed is at least 2 seconds. |
| • | No related HO2S DTCs since code clear. |
| • | The HO2S voltage measures greater than 0.468 volts (486 mV). |
Action Taken When the DTC Sets
The VCM turns on the MIL (Malfunction Indicator Lamp) after 2 consecutive failures.
Conditions for Clearing the MIL/DTC
The VCM turns OFF the MIL after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (the coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature exceeds 71°C (160°F) during that same ignition cycle) or the scan tool clearing feature has been used.
Diagnostic Aids
Check the following items:
| • | The fuel pressure: If the pressure is too high, the system will run rich. The VCM can compensate for some increase; however, if the pressure gets too high, the DTC P0138 may set. Refer to Fuel System Pressure Test . |
| • | A rich injector. Preform a poppet nozzle test. Refer to Fuel Injector Balance Test . |
| • | A leaking injector. Refer to Fuel Injector Balance Test . |
| • | Fuel contaminated oil |
| • | EVAP canister purge: Check for a fuel saturation. If full of fuel, check the canister control and hoses. |
| • | Leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for fuel. |
| • | The TP sensor: An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving. |
| • | False rich indication due to silicon contamination of the heated oxygen sensor. A DTC P0138 accompanied by a lean driveability conditions and a powdery white deposit on the sensor indicates a false rich indication. |
| • | Faulty HO2S: the HO2S is internally shorted, the HO2S voltage displayed on a scan tool will be over 1.0 volt (1000 mV). Try disconnecting the HO2S, if the displayed voltage goes from over 1000 mV to around 450 mV, replace the HO2S. |
Never solder the HO2S wires. For proper wire and connector repair. Refer to Wiring Repairs in Electrical Diagnosis.
Test Description
The number below refers to the step number on the diagnostic table.
-
This test determines if the conditions exist in order to set the DTC P0152.
-
If the HO2S is internally shorted, the HO2S voltage displayed on a scan tool will be over 1.0 volt (1000 mV.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This data will be lost when the clear DTC Information function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
Does the scan tool data display indicate HO2S voltage fixed above the specified value? | 1200 RPM 0.976 V (976 mV) | |||
Does the scan tool data display indicate the HO2S voltage near the specified value? | 400 mV | |||
4 | The DTC is intermittent. Are any additional DTCs set? | -- | Go to the applicable DTC table | Go to Diagnostic Aids |
5 |
Was the HO2S Signal circuit shorted? | -- | ||
6 | Replace the HO2S. Refer to Heated Oxygen Sensor (HO2S) Replacement . Is the action complete? | -- | -- | |
7 | Replace the VCM. Important: When replacing the VCM, the new VCM will need to be programmed. Refer to VCM Replacement/Programming . Is the action complete? | -- | -- | |
8 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
9 | Use the scan tool in order to display the Capture Info and the Review Capture Info functions. Are there any DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
