Circuit Description
The heated oxygen sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.
The HO2S contains a heater that is necessary in order to quickly warm the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches operating temperature, the control module monitors the HO2S bias, or reference, voltage. It also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or "lean-out" the mixture, when the HO2S detects a rich exhaust condition.
Certain vehicle models utilize an oxygen sensor behind the catalytic converter in order to monitor catalyst efficiency.
This diagnostic trouble code (DTC) determines if the HO2S is functioning properly. It checks for an adequate number of HO2S voltage transitions above and below the bias range of 300-600 mV. This DTC sets when the vehicle control module (VCM) fails to detect a minimum number of voltage transitions above and below the bias range during the test period. Possible causes of this DTC are:
| • | An open or a short to voltage on either the HO2S signal or HO2S low circuits |
| • | A malfunctioning HO2S |
| • | A problem in the HO2S heater or its circuit |
| • | A faulty HO2S ground |
This DTC determines if the HO2S is functioning properly by checking for an adequate number of HO2S voltage transitions above and below the bias range of 300-600 mV. This DTC sets when the VCM fails to detect a minimum number of voltage transitions above and below the bias range during the test period. Possible causes of this DTC are listed below.
| • | An open or a short to voltage on either the HO2S signal or HO2S low circuits. |
| • | A malfunctioning HO2S. |
| • | A problem in the HO2S heater or its circuit. |
| • | A poor HO2S ground. |
Conditions for Running the DTC
| • | No active TP sensor DTCs |
| • | No active EVAP system DTCs |
| • | No active IAT sensor DTCs |
| • | No active MAP sensor DTCs |
| • | No active ECT sensor DTCs |
| • | No active MAF sensor DTCs |
| • | No active misfire DTCs |
| • | No intrusive test in progress |
| • | No device controls active |
| • | The decel fuel cut-off mode (DFCO) is not active |
| • | The system voltage is more than 9 volts |
| • | The engine run time is more than 2 minutes |
| • | The system is in closed loop |
| • | The ECT is more than 80°C (176°F) |
| • | The MAF is 15 g/s or more |
| • | DTC P0155 not active |
Conditions for Setting the DTC
The HO2S sensor voltage is between 350-550 mV for more than 40 seconds.
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) if a failure is detected during 2 consecutive key cycles. |
| • | The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tool Freeze Frame/Failure Records. |
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
Important: Never solder the HO2S wires. For proper wire and connector repair, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Check for the following conditions:
An exhaust leak typically within 30.5 cm (12 in) up-stream of the HO2S. Refer to Exhaust System Inspection in Engine Exhaust.
A malfunctioning HO2S heater or heater circuit
With the ignition ON and the engine OFF, the HO2S voltage displayed on the scan tool should gradually drop to below 0.150 volt, indicating that the heater works properly. If not, disconnect the HO2S and connect a test lamp between the terminals C and D. If the test lamp does not light, repair the open in the HO2S ground circuit or the HO2S ignition positive voltage circuit.
The Intermittent Test
Use a scan tool in order to monitor the HO2S signal voltage. Move the related connectors and the wiring harness with a warm engine running at part throttle in Closed Loop. If the failure is induced, the HO2S signal voltage reading changes from its normal fluctuating voltage (above 600 mV and below 300 mV) to a fixed value around 450 mV. This may help to isolate the location of the malfunction.
An intermittent may be caused by any of the following conditions:
| • | A poor connection |
| • | Rubbed through wire insulation |
| • | A broken wire inside the insulation |
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step checks for proper sensor activity. When in Closed Loop fuel control the HO2S voltage should rapidly swing above and below the bias voltage.
-
This step checks the VCM and the HIGH and LOW circuits between the VCM and the HO2S connector for proper operation.
-
This step checks for proper HO2S heater circuit operation up to the HO2S connector.
-
This step checks for proper circuit resistance between the HO2S LOW circuit and VCM ground.
-
This step checks for proper circuit resistance between the HO2S LOW circuit and VCM ground with a COLD sensor. A loose HO2S or poor thread to exhaust electrical contact will cause higher resistance when the HO2S is cold. Although 500 ohms is allowed, typical resistance should be less than 50 ohms.
Step | Action | Value(s) | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool Capture Info to save the Freeze Frame and Failure records for reference. The control module's data is deleted once the Clear Info function is used. Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||||||
Is the HO2S voltage fixed within the voltage range specified? | 350-550 mV | |||||||||||||||
3 | This DTC is intermittent. Are any additional DTCs stored? | -- | Go to the applicable DTC table | Go to Diagnostic Aids | ||||||||||||
Is the voltage less than the specified value? | 20 mV | |||||||||||||||
5 |
Is the resistance of both circuits less than the specified value? | 5ohms | ||||||||||||||
6 |
Did you find a problem? | -- | ||||||||||||||
Is the test lamp ON? | -- | |||||||||||||||
Is the resistance less than the specified value? | 500ohms | |||||||||||||||
9 | Repair the circuit that measured high resistance. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||||||
10 | Repair the HO2S heater ignition positive voltage or ground circuit. Refer to Wiring Repairs in Wiring Systems. Probable causes include the following:
Is the action complete? | -- | -- | |||||||||||||
11 | Repair the high resistance between the HO2S LOW circuit and the VCM ground circuit. Refer to Wiring Repairs in Wiring Systems. Probable causes include the following:
Is the action complete? | -- | -- | |||||||||||||
12 |
Did you find a problem? | -- | ||||||||||||||
Is the resistance less than the specified value? | 500ohms | |||||||||||||||
14 |
Is the resistance less than the specified value? | 500ohms | ||||||||||||||
15 | Replace the HO2S sensor. Refer to Heated Oxygen Sensor Replacement . Is the action complete? | -- | -- | |||||||||||||
16 |
Is the action complete? | -- | -- | |||||||||||||
17 |
Does the scan tool indicate the diagnostic Passed? | -- | ||||||||||||||
18 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
