Circuit Description
The VCM supplies a voltage of about 0.45V (450 mV) between the HO2S High and the HO2S Low circuits. (If measured with a 10 megohm digital voltmeter, this may read as low as 0.32V). The HO2S (Bank 2 Sensor 2) varies the voltage within a range of about 1.0V (1000 mV) if the exhaust is rich, down to about 0.10V (100 mV) if the exhaust is lean. DTC P0157 is a type A DTC.
The sensor is like an open circuit and produces no voltage when it is below about 360°C (600°F). DTC P0151, P0152 or an open sensor circuit or cold sensor causes Open Loop operation. DTC P0157 determines if the HO2S sensor or circuit is shorted to low by checking for a lean condition during steady throttle and power enrichment (PE).
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 |
| • | System voltage is at least 9 V |
Test Enable Lean Test
| • | Closed Loop |
| • | No fuel injectors turned OFF |
| • | Air/fuel ratio is at least 14.5 but less than 14.8 |
| • | TP is greater than 3.5% but less than 99% |
| • | Above met for 5 seconds |
| • | HO2S (Bank 2 Sensor 2) DTCs not active since code clear |
| • | HO2S (Bank 2 Sensor 2) voltage is less than (0.026 V) 26 mV for (5.7L) and less than (0.17 V) 17 mV) for (7.4L) |
Test Enable PE Lean Test
| • | Closed Loop |
| • | Power Enrichment mode active |
| • | High speed fuel cutoff not active |
| • | DTC P0157 not active since code clear |
| • | Time elapsed since test enable is at least 1 second |
| • | HO2S (Bank 2 Sensor 2) voltage is less than 399 mV (0.399 V) |
Action Taken When the DTC Sets
A current DTC P0157 set will cause the system to operate in Open Loop. With DTC P0157 set, the MIL (Malfunction Indicator Lamp) will be turned ON the first test failure.
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
Using the scan tool, observe the L.T. fuel trim values at different RPM and air flow conditions. The scan tool also displays the fuel trim cells, so the L.T. fuel trim values can be checked in each of the cells to determine when DTC P0157 may have been set. If the conditions for DTC P0157 exist, the L.T. fuel trim values will be around 158 or greater.
Check for the following:
| • | HO2S wire - Sensor pigtail may be mispositioned and contacting the exhaust manifold |
| • | Intermittent ground in the wire between the connector and the sensor |
| • | Poor VCM to engine block ground |
| • | Lean injector(s). Perform Fuel Injector Balance Test for the 5.7L. Perform the Fuel Injector Balance Test for the 7.4L. |
| • | Fuel contamination. Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC P0151 |
| • | Fuel pressure. System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to Fuel System Diagnosis . |
| • | Exhaust leaks. If there is an exhaust leak, the engine can cause outside air to be pulled into the exhaust and past the sensor. |
| • | Vacuum or crankcase leaks can cause a lean condition and/or possibly a high idle. |
| • | If the above are OK, it is a faulty HO2S. |
| • | Never solder the HO2S wires. |
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
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Running the engine at 1200 RPM keeps the HO2S hot to ensure the HO2S remains active and can indicate the exhaust oxygen content accurately.
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Opening the HO2S sensor circuit should result in a displayed voltage between 350 and 550 mV. If the display is still fixed below 350 mV, the fault is a short to ground in the HO2S (Bank 2 Sensor 2) high signal circuit or a faulty VCM.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 |
Important: Before clearing DTC(s) use the scan tool to record Freeze Frame and Failure Records for reference, as 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 data display indicate HO2S (Bank 2 Sensor 2) voltage fixed less than the specified value? | 1200 RPM 0.086 V (86 mV) | |||
Does the scan tool data display indicate HO2S (Bank 2 Sensor 2) voltage within the specified range? | 0.35 - 0.55 V (350 mV - 550 mV) | |||
4 | The DTC is intermittent. If no additional DTCs are stored, refer to Diagnostic Aids. If additional DTCs are stored refer to those Table(s). Are any additional DTCs stored? | Go to the Applicable DTC Table | ||
5 | Check for a short to ground in the HO2S (Bank 2 Sensor 2) signal circuit. Was a problem found? | -- | ||
6 | Repair the circuit as necessary. Is the action complete? | -- | -- | |
7 | Refer to Diagnostic Aids. | -- | -- | -- |
8 | Replace the VCM. Important: If the VCM is faulty, the new VCM must be programmed. Refer to VCM Replacement/Programming (With KS Calibration PROM) . Is the action complete? | -- | -- | |
9 |
Does the scan tool indicate that this diagnostic Ran and Passed? | -- | ||
10 | Using the scan tool, select Capture Info, Review Info. Are any additional DTCs displayed that have not been diagnosed? | -- | Go to the Applicable DTC Table | System OK |
