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 difference in transition times from lean to rich or rich to lean has exceeded a maximum ratio. The VCM monitors the time it takes for the HO2S to transition from a rich (above 600 mV) to a lean (below 300 mV) and vice versa. If either of these times is longer than the other by a calibrated ratio, this DTC will set.
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 tests (i.e., EGR or Catalyst) in progress |
| • | No device control in progress |
| • | The system voltage is between 11.7-18 volts |
| • | DTCs P0151, P0152, P0154, and P0155 not active |
| • | The system is in closed loop |
| • | The ECT is more than 57°C (135°F) |
| • | The engine has been running for more than 75 seconds |
| • | The MAF is between 15-55 g/s |
| • | The engine speed is between 1100-3000 RPM |
| • | EVAP canister purge active |
| • | The above conditions are present for more than 2 seconds |
Conditions for Setting the DTC
The number of lean-to-rich and rich-to-lean transitions within a 100 second sample period were less than a calibrated value
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 repairs, refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Check for the following conditions:
| • | An improperly installed air cleaner outlet duct |
| • | The air cleaner outlet duct for collapsed ducting, restrictions, or a missing or plugged air filter |
| • | Throttle body and intake manifold vacuum leaks |
| • | A damaged or blocked throttle body inlet |
| • | Exhaust system for corrosion, leaks, or loose or missing hardware. Refer to Exhaust System Inspection in Engine Exhaust. |
| • | The HO2S is installed securely and the pigtail harness is not contacting the exhaust manifold or wires. Refer to Heated Oxygen Sensor Replacement . |
| • | HO2S contamination |
| • | The vacuum hoses for splits, kinks, and proper connections |
| • | Excessive water, alcohol, or other contaminants in the fuel. Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | VCM sensor grounds that are clean, tight, and properly positioned |
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 number below refers to the step number on the diagnostic table.
-
HO2S contamination is indicated if multiple Response, Switching or Time Ratio HO2S DTCs are set.
-
The use of leaded fuel may be indicated by the removal or tampering of the fuel filler restrictor.
An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of un-approved silicon RTV engine gasket material or the use of silicon based sprays or fluids within the engine.
If the cause of this contamination is not corrected, the replacement HO2S will also get contaminated.
-
Even small exhaust leaks can cause slow response from the HO2S.
-
An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of un-approved silicon RTV engine gasket material or the use of the use of silicon based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also get contaminated.
-
If the voltage observed in step 8 is less than the range specified a short between the HIGH and LOW circuits or a short between the HIGH circuit and ground is indicated. With the HO2S and the VCM disconnected the resistance between the HIGH and LOW circuits and the resistance between the HIGH circuit and ground should measure infinite.
If the voltage observed in step 8 is greater than the range specified a short between the HIGH circuit and an ignition voltage source is indicated.
-
If the voltage observed in step 9 is not below the voltage specified a high resistance (open) HIGH or LOW circuit is indicated.
Good circuit continuity measures less than 5 ohms with the VCM and sensor disconnected. Measure between the VCM connector and the HO2S connector. Ensure VCM terminal contact is good.
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? | -- | ||||||||||||
In addition to this DTC check for the following HO2S DTCs:
Are any of the above listed HO2S DTCs set? | -- | |||||||||||||
3 | Are any powertrain component DTCs set? | -- | Go to the applicable DTC table | |||||||||||
Has the contamination source been removed and the affected sensors replaced? | -- | -- | ||||||||||||
5 |
Did this DTC run and fail this ignition? | -- | ||||||||||||
6 | This DTC is intermittent. Are any additional DTCs stored? | -- | Go to the applicable DTC table | Go to Diagnostic Aids | ||||||||||
Repair exhaust as necessary. Refer to Exhaust System Inspection in Engine Exhaust. Repair as necessary. Refer to Connector Repairs in Wiring Systems. Did you find a problem? | -- | |||||||||||||
8 |
Is the HO2S voltage within the value range specified? | 351 - 551 mV | ||||||||||||
9 |
Is the HO2S voltage less than the value specified? | 20 mV | ||||||||||||
10 |
Is the test lamp ON? | -- | ||||||||||||
11 | Connect the test lamp between the HO2S heater ignition positive voltage circuit terminal and the HO2S heater ground circuit terminal (both on engine harness side). Is the test lamp ON? | -- | ||||||||||||
HO2S contamination sources include the following:
Is the action complete? | -- | -- | ||||||||||||
Did you find a problem? | -- | Go to Diagnostic Aids | ||||||||||||
Is the action complete? | -- | -- | ||||||||||||
15 | Repair the HO2S Heater ignition positive voltage circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||||
16 | Repair the HO2S Heater ground circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||||
17 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement . Is the action complete? | -- | -- | |||||||||||
18 |
Does the scan tool indicate the diagnostic Passed? | -- | ||||||||||||
19 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
