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P0151 trouble code

P0151 Trouble Code — What to test and how to fix

Possible causes of a P0151 02 Sensor Circuit Low Voltage (Bank 2 Sensor I)

Check these possible causes BEFORE replacing the oxygen sensor

An exhaust leak can cause a P0151 trouble code

1) An exhaust leak can cause too much outside air to enter the exhaust, causing the oxygen sensor to see too much oxygen. See this post to learn how to check for an exhaust leak.

A vacuum leak or air duct leak can cause a P0151

2) There’s a vacuum leak in the engine, allowing too much unmetered air into the engine, resulting in too much oxygen in the exhaust stream.

3) There’s a tear or breach in the air duct from the air filter box to the throttle body, allowing too much unmetered air into the engine, resulting in too much oxygen in the exhaust stream.

Low fuel pressure or clogged fuel injectors can cause a P0151 oxygen sensor code

4) If the fuel pressure is too low not enough fuel is entering the combustion chamber. Combustion ends early because the fuel has been used up leaving too much oxygen in the exhaust

3) Fuel injector(s) are clogged so not enough fuel is entering the combustion chamber. Combustion ends early because the fuel has been used up leaving too much oxygen in the exhaust

Wiring issues can cause a P0151

4) A wiring harness or connector issue is preventing the oxygen sensor from sending the correct voltage to the ECM.

A failed oxygen sensor can set a P0151 code

5) The sensor has failed.

P0151 symptoms

Check Engine Light is on
Lacks power
Poor gas mileage

Understanding more about oxygen sensors

The Engine Control Module (ECM) uses heated oxygen sensor (HO2S) for two purposes: 1) When seated upstream of the catalytic converter, the oxygen sensor or air fuel ratio sensor is used for fuel control. 2) When situated after the catalytic converter, the sensor is used to monitor the catalyst efficiency. The downstream sensor is not used for fuel control

How an oxygen sensor works

The HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. On older style oxygen sensors, the differential generates voltage but the newer air fuel ratio sensors require voltage to operate.

The HO2S must reach approximately 600°F to provide accurate readings. The exhaust can heat the HO2S to that temperature when cruising, but may not be able to maintain the temperature at idle or in winter. So HO2S sensor incorporate an internal heater to maintain temperature and reduce the time needed for the sensor to read.

A consistently low voltage on Bank 2 means the the sensor is seeing too much oxygen and the exhaust mixture is not switching. The code sets after the ECM has tried to correct the situation by adding more fuel to the mixture to correct the low voltage and the low voltage persists.

Criteria needed to set a P0151 code

Carmakers require that certain parameter must be met before the ECM can set this trouble code. These criteria are call ENABLING CRITERIA. Here’s an example of GM’s enabling criteria for a P0151 code

The ECM must be in closed loop status
The Ignition voltage must beween 10–18 volts.
The Fuel Alcohol Content must be less than 90 percent.
The Fuel Tank Level Remaining parameter is greater than 10 percent.
The throttle position indicated angle parameter is between 3–70 percent more than the value observed at idle.
The above conditions are met for 2 seconds.
This diagnostic runs continuously when the above conditions are met.

If all of those criteria are met, a GM ECM will set the P0151 code if the HO2S voltage is less than 200 mV for 165 seconds. Other car makes and models may choose other voltages or time periods.

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