DTC P0151

Circuit Description

The powertrain control module (PCM) supplies a bias voltage of about 450 mV between the heated oxygen sensor (HO2S) high signal and low signal circuits. When measured with a DMM, this may display as low as 350 mV. The oxygen sensor (O2S) varies the voltage within a range of about 1,000 mV when the exhaust is rich, down through about 100 mV when exhaust is lean. The PCM constantly monitors the HO2S signal during Closed Loop operation and compensates for a rich or lean condition by decreasing or increasing injector pulse width as necessary. If the HO2S voltage remains excessively low for an extended period of time, this diagnostic trouble code (DTC) will be set.

Conditions for Running the DTC

DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
The vehicle is in Closed Loop.
The air/fuel ratio is more than or equal to 14.5, but less than or equal to 14.8.
The accelerator pedal position (APP) indicated angle is between 5-40 percent

Conditions for Setting the DTC

The HO2S signal voltage remains less than 175 mV during normal Closed Loop operation.

The HO2S signal voltage remains less than 600 mV during power enrichment.

Action Taken When the DTC Sets

The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Inspect for the following conditions:

Throttle body inlet screen blockage
Low fuel pressure--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 HO2S DTC may set. Refer to Fuel System Diagnosis .
Fuel injectors contaminated or faulty--Refer to Fuel System Diagnosis .
Exhaust leaks--An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Inspect for exhaust leaks that may cause a false lean condition to be indicated.
A faulty mass air flow (MAF) sensor--Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the MAF sensor. Refer to Mass Air Flow (MAF) Sensor Replacement .
Fuel contamination--Water, even in small amounts, can be delivered to the fuel injectors. The 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 .

Refer to Intermittent Conditions

Test Description

The numbers below refer to step numbers on the diagnostic table.

This DTC failing during power enrichment operation, with the throttle position (TP) angle above 60 percent, may indicate a condition described in Diagnostic Aids. If this DTC test passes while the Failure Records conditions are being duplicated, an intermittent condition is indicated. Review the Failure Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
The new PCM must be programmed.

HO2S Low (Pre)

Step	Action	Values	Yes	No
Schematic Reference: Engine Controls Schematics
1	Did you perform the Diagnostic System Check-Engine Controls?	--	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Important During this step, observe all HO2S voltage parameters. If all parameters remain below the specified value, refer to Diagnostic Aids. Operate vehicle within the Conditions for Setting parameters. With a scan tool, observe HO2S voltage parameter. Does the HO2S voltage remain below the specified value?	400 mV	Go to Step 4	Go to Step 3
3	Operate vehicle within Failure Records conditions. Does the scan tool indicate this DTC failed this ignition?	--	Go to Step 4	Go to Diagnostic Aids
4	Disconnect the HO2S. Turn ON the ignition, with the engine OFF. Does scan tool indicate HO2S voltage less than the specified value?	400 mV	Go to Step 5	Go to Step 7
5	Test the HO2S high signal circuit for a short to ground or a short to the sensor ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	--	Go to Step 8	Go to Step 6
6	Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement . Did you complete the replacement?	--	Go to Step 8	--
7	Replace the HO2S. Refer to Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 , Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 , or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 . Did you complete the replacement?	--	Go to Step 8	--
8	Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Operate the vehicle within the Conditions for Running the DTC, as specified in the supporting text. Does the DTC run and pass?	--	Go to Step 9	Go to Step 2
9	With a scan tool, observe the Stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed?	--	Go to Diagnostic Trouble Code (DTC) List	System OK