Schematic Reference: Engine Controls Schematics
Circuit Description
The heated oxygen sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV under normal operating conditions. The engine control module (ECM) monitors this voltage and determines if the exhaust is indicating a lean or rich fuel mixture. If the voltage input at the ECM is about 100 mV the fuel mixture is lean, and if the voltage input is about 900 mV the fuel mixture is rich. The ECM constantly monitors the HO2S signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing fuel injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended period of time, a DTC P0131 will be set.
Conditions for Running the DTC
| • | Engine coolant temperature (ECT) was greater than -7°C (19°F) at engine start up. |
| • | Intake air temperature (IAT) was greater than -7°C (19°F) at engine start up. |
| • | Barometric pressure is greater than 75 kPa. |
| • | Vehicle speed is above 56 km/h (35 mph) for 2 minutes and then idled for 1 minute. |
Conditions for Setting the DTC
Maximum voltage average is less than 500 mV and minimum voltage average is less than 70 mV while engine is idling per the previous step.
OR
ECM internal voltage check of the HO2S 1 circuit while idling, indicates a higher than expected voltage drop.
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) illuminates after two consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The ECM records the operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns off after three consecutively passing cycles without a fault present. |
| • | A History DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | DTCs can be cleared by using a scan tool or by disconnecting the ECM battery feed. |
Diagnostic Aids
A DTC P0131 is more likely to set during city driving. Avoid operating the vehicle on the highway when validating a DTC P0131.
Check for any of the following conditions:
| • | If any DTCs other than a Fuel Trim (P0171, P0172), Misfire (P0300 to P0304), EGR (P0400) and HO2S 1 (P0131 to P0134) are set, diagnosis those DTCs first. |
| • | Misrouted heated oxygen sensor wiring. The sensor pigtail may be mis-positioned and contacting the exhaust system. |
| • | Correct fuel pressure. The fuel system can go lean if the fuel pressure is too low and a DTC P0131 may set. |
| • | Lean fuel injectors. |
| • | 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 and flowed past the HO2S, causing a DTC P0131 to set. Check for exhaust leaks near the HO2S. |
| • | A shorted HO2S. An HO2S that is shorting to ground, the HO2S reading displayed on the scan tool will be under 0.1 volts. Water contamination of the HO2S can cause a low HO2S voltage to be indicated. Inspect whether the DTC P0131 occurs when driving through standing water or when operating the vehicle in wet driving conditions. Replace the HO2S if the sensor is adversely affected by moisture. |
| • | A contaminated HO2S. Silicon contamination of the HO2S can cause a low HO2S voltage to be indicated. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is found, determine the cause and correct the condition before replacing the HO2S. |
| • | Fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. Too much alcohol in the fuel can also cause a lean exhaust indication |
An intermittent malfunction may be caused by a fault in the HO2S sensor circuit. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connector locks. |
| • | Improperly formed or damaged terminals. |
| • | Faulty terminal to wire connections. |
| • | Physical damage to the wiring harness. |
| • | A broken wire inside the insulation. |
| • | Corrosion of electrical connections, splices, or terminals. |
If the DTC P0131 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
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The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step checks whether the HO2S is operating within the normal range. The vehicle must be at operating temperature.
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This step checks whether the malfunction that caused the DTC P0131 is still present. Driving the vehicle under these conditions will verify that the fault is present. The DTC P0131 diagnostic can be monitored on the scan tool under the MIL/System Status selection of System Information. When the HO2S test displays a YES status (indicating that the purge system diagnostic is completed) check for a DTC P0131 in the Last Test Failed screen of the scan tool. If there is no DTC P0131 displayed the HO2S system diagnostic has run and passed, indicating that no malfunction was present this time. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the system diagnostics being performed. Do not forget that the MIL/System Status tests only indicate that the test has run, not whether the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the outcome of the diagnostic test involved.
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This step checks the HO2S signal circuit. The DMM supplies a safe (low current) voltage to the ECM HO2S signal circuit when connected to B+. The scan tool should display near the high limit (1000-1275 mV) of the HO2S input circuit, if the ECM and the signal wire are OK.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||
Does the HO2S 1 voltage switch below and above the specified value? | 300 mV to 600 mV | |||||||
Does the HO2S 1 voltage switch below and above the specified value? | 300 mV to 600 mV | Fault not Present-Go to Diagnostic Aids | ||||||
4 |
Is the test lamp illuminate? | -- | ||||||
Does the scan tool indicate a HO2S 1 voltage above the specified value? | 1000 mV | |||||||
6 |
Was a repair necessary? | -- | ||||||
7 |
Was a repair necessary? | -- | ||||||
8 | Replace the HO2S 1. Refer to Heated Oxygen Sensor Replacement. Is the action complete? | -- | -- | |||||
9 | Replace the ECM. Refer to Engine Control Module Replacement. Is the action complete? | -- | -- | |||||
10 |
Are any DTCs displayed on the scan tool? | -- | Go to the applicable DTC table | System OK |
