Schematic Reference: Refer to Engine Controls Schematics
Circuit Description
The heated oxygen sensor (HO2S) produces a voltage that varies between 0.1 volts and 0.9 volts under normal operating conditions. The powertrain control module (PCM) monitors this voltage and determines if the exhaust is lean or rich. If the voltage input at the PCM is about 0.1 volts the exhaust is lean, and if the voltage input is about 0.9 volts the exhaust is rich. The PCM constantly monitors the O2S 1 signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing injector pulse width as necessary.
In normal operation the O2S 1 sensor will become active and go into closed loop soon after the engine is running. If the PCM fails to receive a rich (high voltage) signal from the O2S 1 sensor, after the engine cooling system has reached operating temperatures, a DTC P0125 can set.
Conditions for Running the DTC
| • | The engine coolant temperature is at least 40°C (140°F). |
| • | Engine has been running for at least 180 seconds. |
| • | Engine speed is 1,400 RPM or more |
| • | Vehicle speed is between 40-100 km/h (25-62 MPH) |
| • | The throttle valve (TP sensor position) is open. |
Conditions for Setting the DTC
O2S 1 output does not indicate RICH for over 1.5 minutes.
Action Taken When the DTC Sets
| • | The PCM illuminates the MIL during the second key cycle in which the DTC sets. |
| • | The PCM stores the conditions which were present when the DTC set as Freeze Frame Data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present. |
| • | The DTC clears after 40 consecutive warm-up cycles have occurred without a fault. |
| • | A DTC can be cleared by using the scan tool Clear DTC Information function. |
Diagnostic Aids
The scan tool output control feature, Fuel Injector Modification, can be used at idle to check oxygen sensor performance. Increasing the Fuel Injector Modification Commanded State to 25% should cause the O2S 1 voltage to exceed 800 mv. Decreasing the Fuel Injector Modification Commanded State to -12% should cause the O2S 1 voltage to drop below 100 mv. An oxygen sensor that fails to respond accordingly may be faulty or contaminated. When performing Fuel Injector Modification with the scan tool, the Loop Status will remain OPEN LOOP and the Short Term FT will be fixed at 0% and 128 COUNTS.
Check for any of the following conditions:
| • | Damaged oxygen sensor wiring. The sensor pigtail may be improperly routed and contacting the exhaust system. |
| • | Incorrect fuel pressure. A condition that causes a lean exhaust can cause DTC P0125 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 P0125 may be set. Refer to Fuel System Diagnosis . |
| • | Lean fuel injectors. Refer to 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 P0125 to set. Check for exhaust leaks near the O2S 1 sensor. |
| • | Fuel contamination. Water, even in small amounts, can be delivered to the fuel injectors. 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 . |
An intermittent malfunction may be caused by fault in the HO2S circuit sensor electrical circuit. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connectors 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. |
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
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The 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 verifies whether the malfunction is a hard failure or an intermittent.
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This step verifies whether the PCM is able to receive a signal from the O2S 1. The DMM is used as a low current voltage source taking the place of the HO2S.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||
2 |
Does the O2S 1 voltage vary within the specified range? | 200-900 mV | ||
Does a DTC P0125 set? | -- | Fault Not Present-Go to Diagnostic Aids | ||
Is the O2S 1 voltage greater than the specified value? | 1155 mV | |||
5 |
Does the test lamp illuminate? | -- | ||
6 |
Was a repair necessary? | -- | ||
7 | Repair the open in the O2S 1 ground circuit between the O2S 1 and the G106. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
8 | Replace the O2S 1. Refer to Oxygen Sensor Replacement . Is the action complete? | -- | -- | |
9 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | -- | |
10 |
Are any DTCs displayed on scan tool? | -- | Go to the applicable DTC table | System OK |
