Understanding Oxygen Sensor Codes: Myths and Facts

Troubleshooting Oxygen Sensor Codes: What You Need to Know

Oxygen sensors and air-fuel ratio sensors have two separate

Learn what the oxygen sensor codes mean

components; the heater and the sensor. When you get an oxygen sensor code, that does NOT automatically mean the sensor is bad. In fact, 80% of all oxygen sensors returned under warranty are perfectly good. In other words, the sensors were telling the truth and the cause of the code was due to an incorrect air fuel mixture or an air or exhaust leak.

The same applies to oxygen sensor heater codes. DIYers automatically replace the sensor when they see an oxygen sensor heater code. However, with oxygen sensor heater-related codes, the problem can be caused by a blown fuse, an open wire, a bad connection in the oxygen sensor connector, or a bad heater inside the sensor.

Deciphering The Oxygen Sensor’s  Location

A Bank 1 sensor is located on the engine bank that holds Cylinder #1
A Bank 2 sensor is located on the engine bank that is opposite the bank holding Cylinder #1
Sensor 1 is before the catalytic converter and is used by the ECM to calculate air/fuel mixtures.
Sensor 2 is located after the catalytic converter and is used to gauge catalytic converter efficiency.

What oxygen sensor readings should look like

Sensor 1 readings should be rapidly changing from high to low voltage a minimum of 8 times per second.

Sensor 2 readings should be fairly steady

Oxygen sensor codes and their meaning

P0151 02 Sensor Circuit Low Voltage (Bank 2 Sensor I)

A consistently low voltage on Bank 2 means the sensor is seeing too much oxygen in the exhaust. 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

Possible causes of a P0151 code

Check these possible causes BEFORE replacing the oxygen sensor

1) An exhaust leak can cause too much outside air to dilute the exhaust, causing the oxygen sensor to see too much oxygen. Check for an exhaust leak ahead of the upstream oxygen sensor.
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 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.
4) Fuel pressure is too low, 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
5) 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
6) A wiring harness or connector issue is preventing the oxygen sensor from sending the correct voltage to the ECM.
7) The sensor has failed.

P0151 symptoms

P0152 02 Sensor Circuit High Voltage (Bank 2 Sensor 1)

This code is the opposite of a P0151. A consistently high voltage on Bank 2 means the sensor is seeing too little oxygen and the exhaust mixture is not switching. The code is set after the ECM has tried to correct the situation by subtracting more fuel from the mixture to correct the high voltage, which persists.

Possible causes of a P0152 code

Check these possible causes BEFORE replacing the oxygen sensor

1) Excessive fuel pressure causing the injectors to add more fuel than required. Check fuel pressure readings and test fuel pressure regulator
2) Leaking fuel injectors that allow more fuel into the cylinder
3) Frayed, damaged, or shorted wiring
4) Faulty connector or wiring harness (short to sensor from sensor heater)
5) A P0152 is rarely caused by a faulty oxygen sensor

P0152 symptoms

Poor gas mileage
Rough running engine
Emissions failure

P0153 02 Sensor Circuit Slow Response (Bank 2 Sensor 1)

The ECM rapidly changes the air/fuel mixture, so the oxygen sensor must detect the rapid changes. The oxygen sensor must be capable of detecting a minimum of 8 changes per second. This code means the sensor is “lazy” and can’t detect rapid changes.

The most likely causes of P0153 trouble code

Exhaust leak that’s diluting the exhaust airflow
Contaminated O2 sensor (oxygen sensors CANNOT be cleaned. Don’t even try)

Fix for P0153

Check for exhaust leak and repair
Replace oxygen sensor

P0154 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)

No activity can be caused by a failed sensor or an open or short in the wiring harness or connector

Diagnose P0154

Using a wiring diagram and meter, check for heater and sensor voltage at the oxygen sensor connector.

If you don’t see the correct voltage or find missing ground, trace the wiring harness to correct the problem. If you do see the correct voltage and ground, the sensor is bad and must be replaced.

P0155 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

An oxygen sensor must reach 600°F before it can accurately read the oxygen level in the exhaust. The engine exhaust can heat it to that temperature while driving but can’t keep it at that temperature during idle or extremely low ambient temperatures.

So modern oxygen sensors incorporate a heater inside the sensor to maintain a 600° F temperature. A P0155 trouble code means there’s a problem with the heater

The most common causes of P0155 trouble code

Blown fuse for the oxygen sensor heater circuit
Open the wiring harness
Corrosion or poor connection in the oxygen sensor connector
Failed heater in the oxygen sensor

P0156 02 Sensor Circuit Malfunction (Bank 2 Sensor 2)

The ECM expects to see a certain minimum voltage. If it doesn’t, it sets this code.

Most common causes of P0156

Exhaust leak
Wiring harness or connector issues
Faulty oxygen sensor

P0157 02 Sensor Circuit Low Voltage (Bank 2 Sensor 2)

See P0151

P0158 02 Sensor Circuit High Voltage (Bank 2 Sensor 2)

See P0152

P0159 02 Sensor Circuit Slow Response (Bank 2 Sensor 2)

See P0153

P0160 02 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)

See P0154

P0161 02 Sensor Heater Circuit Malfunction (Bank 2 Sensor 2)

See P0155

©, 2022 Rick Muscoplat