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
The following conditions will set the DTC:
| • | No TP sensor DTCs |
| • | No EVAP DTCs |
| • | No IAT sensor DTCs |
| • | No MAP sensor DTCs |
| • | No ECT sensor DTCs |
| • | No fuel trim DTCs |
| • | No EGR DTCs |
| • | No intrusive test in progress |
| • | No device controls active |
| • | The system voltage measures at least 9.0 volts |
Test Enable Lean Test
| • | Closed Loop low MAP not active |
| • | Closed Loop |
| • | No fuel injectors turned off |
| • | Air to fuel ratio is at least 14.5 but less than 14.8 |
| • | The TP is greater than 5% but less than 50% |
| • | The above met for 5 seconds |
| • | Related HO2S DTCs not active since code clear |
Test Enable PE Lean Test
| • | A Closed Loop |
| • | The Power Enrichment mode active |
| • | The high speed fuel cutoff not active |
| • | Related HO2S DTCs not active since code clear |
| • | Time elapse since test enable is at least 1 second |
Test Enable Rich Test
| • | A Closed Loop |
| • | Air to fuel ratio is at least 14.5 but less than 14.8. |
| • | The TP is greater than 5% but less than 50%. |
| • | The above must meet for 5 seconds. |
| • | HO2S (Bank 1, Sensor 2) DTCs not activate since the code clear |
| • | The HO2S (Bank 1, Bank 2) voltage measures greater than 0.993 volts (993 mV). |
Test Enable DFCO Rich Test
| • | Decel Fuel cutoff mode is active. |
| • | A Closed Loop |
| • | The time elapsed is at least 2 seconds. |
| • | HO2S (Bank 1, Sensor 2) not active since the clear code. |
| • | The HO2S (Bank 1, Sensor 2) voltage measures greater than 0.800 volts (800 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 Diagnosis . |
| • | 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 (Bank 1, Sensor 2): the HO2S (Bank 1, Sensor 2) is internally shorted, the HO2S (Bank 1, Sensor 2) voltage displayed on a scan tool will be over 1.0 volt (1000 mV). Try disconnecting the HO2S (Bank 1, Sensor 2), if the displayed voltage goes from over 1000 mV to around 450 mV, replace the HO2S (Bank 1, Sensor 2). |
Never solder the HO2S wires.
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 |
Important: Before clearing the DTCs use the scan tool to record the Freeze Frame and the Failure Records data. This data will be lost when the Clear Info function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
Does the scan tool display indicate the HO2S (Bank 1, Sensor 2) sensor voltage fixed above the specified value? | 1200 RPM 0.993 V (993 mV) | |||
3 |
Does the scan tool display indicate the HO2S (Bank 1, Sensor 2) voltage less than the specified value? | 0.20 V (200 mV) | ||
4 | The DTC is intermittent. If no additional DTCs are stored, refer to the Diagnostic Aids. If additional DTCs are stored, refer to those tables. Are any additional DTCs stored? | -- | Go to The Applicable DTC Table | |
5 | Refer to the Diagnostic Aids | -- | -- | -- |
6 | Replace the VCM. Important: If the VCM is faulty, reprogram the VCM. Refer to VCM Replacement/Programming . Is the action complete? | -- | -- | |
7 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
8 | Using the scan tool, select the Capture Info and the Review Info. Are any DTCs displayed that have not been diagnosed? | -- | Go to The Applicable DTC Table | System OK |
