What is a P0151 trouble code and how do you fix it?

The 5 most common causes of a P0151 trouble code

A P0151 trouble code is defined as: 02 Sensor Circuit Low Voltage (Bank 2 Sensor I).

Where is bank 2 Sensor 1 and what does low voltage mean

Bank 1 on a “V” engine is the bank with cylinder #1, so bank 2 is the other bank. Sensor 1 is the oxygen sensor before the catalytic converter. So this code means the oxygen sensor on the exhaust from bank 2 and ahead of the converter is constantly reporting a low voltage. Normally, an oxygen sensor reports rapidly changing high and low voltages aa the oxygen in the exhaust constantly varies.

Don’t assume the oxygen sensor is bad! That’s a common DIY mistake. Check these items first

An exhaust leak can cause a P0151 trouble code

An exhaust leak can cause too much outside air to enter the exhaust, causing the oxygen sensor to see too much oxygen. Extra oxygen causes a low voltage at the oxygen sensor.  See this post to learn how to check for an exhaust leak.

A vacuum leak or air duct leak can cause a P0151

A vacuum leak in the engine can allow too much unmetered air into the engine, resulting in too much oxygen in the exhaust stream.

A tear or broken plastic tubing in the air duct from the air filter box to the throttle body can allow 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

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

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

The sensor has failed or become contaminated.

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

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.

©, 2022 Rick Muscoplat