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 is designed to detect an HO2S voltage that remains at a high (rich) voltage for more than a specified number of seconds during the test conditions. This DTC is set under the following conditions:
| • | There is an HO2S circuit problem that results in a false rich exhaust condition. |
| • | The HO2S is correctly detecting the rich air/fuel ratio resulting from either a fuel control or emission system problem. |
Conditions for Running the DTC
| • | No active TP sensor DTCs |
| • | No active EVAP 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 active |
| • | The system voltage is between 11.7-18 volts |
Rich Test
| • | System in closed loop |
| • | The air/fuel ratio is between 14.5-14.8 |
| • | The throttle position is between 0-50 % |
Decel Fuel Cut-off (DFCO) Rich Test
| • | DFCO mode active |
| • | System in closed loop |
| • | The elapsed time since test enabled is greater than 2 seconds |
Conditions for Setting the DTC
Rich Test
The O2 sensor voltage is greater than 976 mV for more than 40 seconds.
DFCO Test
The O2 sensor voltage is greater than 468 mV for more than 30 seconds.
Action Taken When The DTC Sets
A current DTC P0151 causes the system to operate in open loop. With the DTC P0151 set, the MIL (Malfunction Indicator Lamp) turns ON at the first test failure.
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
Check the following items
| • | Fuel pressure. System will go rich if pressure is too high. The VCM can compensate for some increase. However, if it gets too high, DTC P0158 may be set. Refer to Fuel System Diagnosis . |
| • | Rich injector. Perform the Fuel Injector Balance Test . |
| • | Check for fuel contaminated oil. |
| • | EVAP canister purge. Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to Evaporative Emission Control System Diagnosis . |
| • | Check for leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for fuel. |
| • | TP sensor. An intermittent TP sensor output will cause the system to go rich due to a false indication of the throttle moving. |
| • | False rich indication due to silicon contamination of the HO2S. This will be indicated by DTC P0158 accompanied by lean driveability conditions and a powdery white deposit on the sensor. |
| • | Faulty HO2S (Bank 2 Sensor 2). If HO2S (Bank 2 Sensor 2) is internally shorted the HO2S (Bank 2 Sensor 2) voltage displayed on a scan tool will be over 1 V. Try disconnecting the HO2S (Bank 2 Sensor 2); if the displayed voltage goes from over 1000 mV to around 450 mV, replace HO2S (Bank 2 Sensor 2). |
| • | Never solder the HO2S wires. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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In order to determine if the engine is lean during power enrichment operate the vehicle under higher throttle/acceleration conditions while monitoring scan tool HO2S voltage and the power enrichment parameter. A lean condition will cause HO2S voltage to be less than 598 mV during power enrichment.
-
Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor check for lean conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S needs replacement or a lean condition exists only on the suspect HO2S cylinder bank.
Step | Action | Value(s) | Yes | No | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool Capture Info function to save the Freeze Frame and the Failure Records for reference. The control module's data is deleted once the Clear DTC Information function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||||||||||||||
2 |
Is the HO2S voltage fixed less than the value specified? | 86 mV | ||||||||||||||||||||||
The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:
Repair any engine conditions as necessary. Did you perform a repair? | -- | Go to Diagnostic Aids | ||||||||||||||||||||||
4 |
Is the HO2S voltage within the specified value range? | 351 mV-551 mV | ||||||||||||||||||||||
The HO2S may be detecting a lean exhaust condition. Check for one of the following conditions:
Repair any engine conditions as necessary. Did you perform a repair? | -- | |||||||||||||||||||||||
6 |
Is the resistance at the specified value on both tests? | ∞ | ||||||||||||||||||||||
7 | Repair the HO2S signal HIGH circuit shorted to ground or shorted to the HO2S LOW circuit. Refer to Wiring Repairs . Is the action complete? | -- | -- | |||||||||||||||||||||
8 | Replace the HO2S. Refer to Heated Oxygen Sensor Replacement . Is the action complete? | -- | -- | |||||||||||||||||||||
9 |
Is the replacement complete? | -- | -- | |||||||||||||||||||||
10 |
Does the scan tool indicate the diagnostic Passed? | -- | ||||||||||||||||||||||
11 | Does the scan tool display any additional undiagnosed DTCs? | -- | Go to the applicable DTC table | System OK |
