Oxygen Sensor Codes Keep Coming Back? 3 Reasons Why

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It can be frustrating to see oxygen sensor codes keep coming back. There are no codes that specifically tell that your oxygen sensor or related parts need to be replaced, hence it is important to check the entire chain of things that are responsible for your specific error code.

If your oxygen sensor codes keep coming back after you replaced the sensor, it’s highly probable your O2 sensor wasn’t at fault to begin with. The problem could be anything from bad spark plugs to bad wiring. Hence it is important to check the entire group of parts that could be responsible for your specific error code.

1. The Error Code Didn’t Reset

If you’ve verified that the error code is the same, you may still be getting the codes because the computer hasn’t realized that the problem is solved. Usually all you have to do to fix this is:

  • Drive for a bit for the computer to re-calibrate and recognize that the problem that was causing the error codes has gone away. 
  • Try unplugging your battery for about 30 minutes, but note that disconnecting the battery will likely erase other data as well. (Note: Newer cars use non-violate storage for storing data like emissions error codes, so disconnecting the battery might not help)
  • If you are certain the codes are false alarms, use an OBD2 scanner to reset the codes.

2. Your Oxygen Sensor Wasn’t At Fault

As we said, most of time, if your oxygen sensor codes keep coming back it’s because the O2 sensor wasn’t at fault in the first place, and you probably replaced it for nothing. To prevent this from happening again, use our chart below to understand what those O2 sensor codes actually mean and what are the possible causes of them.

TIP: Never assume a certain part’s replacement is necessary, just because the error code includes its name in the description

CodeDescriptionPossible Causes
P0150
P0156
O2 Sensor Circuit Malfunction– Broken sensor element.
– Sensor disconnected.
– Shorted wiring.
– Catastrophic failure of sensor due to thermal shock.
P0151
P0157
P0163
O2 Sensor Circuit Low Voltage– Short in wiring between sensor ground and signal wire.
– Silicone or ethylene glycol poisoning of the air reference electrode.
P0152
P0158
P0164
O2 Sensor Circuit High Voltage– Short in wiring between heater circuit and signal wire.
– Sensor immersed in water.
– Silicone or ethylene glycol poisoning of the sensing electrode.
P0153
P0159
P0165
O2 Sensor Circuit Slow Response– Sensor electrode protective coated with carbon.
– Silicone poisoning.
– Ethylene glycol poisoning.
– Failed sensor heater.
– Heater circuit fuse.
P0154
P0160
P0166
O2 Sensor Circuit No Activity Detected– Short in wiring between sensor ground and signal wire.
– Silicone or ethylene glycol poisoning of the air reference electrode.
P0155
P0161
P0167
O2 Sensor Heater Circuit Malfunction– Sensor heater shorted or open.
– Replacement sensor installed with incorrect heater current values.
– Open or shorted electrical connections. Heater circuit fuse.
P0156
P0162
O2 Sensor Circuit Sensor Malfunction– Broken sensor element.
– Sensor disconnected.
– Shorted wiring.
– Catastrophic failure of sensor due to thermal shock.

Note: If you received a different error code, a quick google search about the code should do the trick.

3. You Replaced The Wrong Oxygen Sensor

On a modern car, there are two O2 sensors. The first is used to adjust the fuel mixture and the second one checks to make sure the first O2 sensor and the catalytic converter are functioning properly.

It is possible the second sensor went bad fooling the computer into thinking the first O2 sensor was failing. Or else, it could be a wiring problem causing a misreading. It’s also possible, though less likely, that the computer is being fooled by another sensor that is overriding the O2 sensor input.

Related: How To Tell If a Catalytic Converter Is Clogged

Oxygen Sensor Codes Keep Returning? How To Test An Oxygen Sensor

If you want to test whether your o2 sensor is actually at fault for your error codes coming back, you can simply use a multimeter or an OBD2 scanner

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Depending on which error code and bank you got, you will need to remove the sensor in several locations because there is more than one. The first is used to adjust the fuel mixture, while the second one checks to make sure the first O2 sensor and the catalytic converter are functioning properly.

Although some cars can have as high as 4 oxygen sensors. This is because any car made in 1996 and after are required to have a second oxygen sensor that is located below the catalytic converter, to monitor the efficiency, and indicate if it clogged or damaged.

Any of these could be the culprit in causing your oxygen sensor codes to keep returning.

Air Fuel Oxygen Sensor

Scotty Kilmer has a good video showcasing how it’s done. Of course, you can check out his car forum here

Text version:

Scotty Kilmer with oxygen sensor socket

To get the oxygen sensor off you’ll need an oxygen sensor socket which slides on the cheater bar, which will give you plenty of leverage to get the oxygen sensor loose.

oxygen sensor location on engine
  • At first, locate the electrical connection, from under the car.
wiring connector
  • Secondly, unplug the socket.
cheater bas being used to remove oxygen sensor
  • Now, use the cheater bar with an oxygen sensor socket to loosen the sensor.
unscrew sensor by hand
  • Once loose, unscrew the oxygen sensor by hand.
oxygen sensor detached and removed
  • Last, and without expending too much blood, sweat, or tears…you’re done!

Other Location

In the event that you couldn’t find the oxygen sensor, consulting your owner’s manual or doing a google search about your car will show you where your oxygen sensor is located. But don’t worry, even if it was in an unusual spot, it shouldn’t be that much different from what we showed above, in fact, it might be easier and more intuitive!

Now that we have the sensor out, we can move on with our diagnosis on why the oxygen sensor codes keep coming back. But first, you must know which type you have.

Before we start the testing procedures, your oxygen sensor might have 1,2,3 or 4 wires, with

  • 1 Wire: One signal wire, grounded by the exhaust pipe in the chassis, sends the voltage reading to the ECU, which then outputs the oxygen sensor codes.
  • 2 Wires: One signal wire, and the wire is going to be a sensor ground wire, and is more reliable than grounding it with the chassis of the exhaust pipe
  • 3 Wires: One signal wire, grounded by the exhaust pipe in the chassis, and the extra wire being the heater, which helps your oxygen sensor reach operating temperatures much faster because your oxygen sensor won’t produce any voltage unless it reaches 600 degrees.
  • 4 Wires: One signal wire. One ground wire, the remaining two wires for heating.

Generally, the black wire is the signal wire, the white wire is the heater and the grey wire is the ground.

A 4-wire oxygen sensor will be used as an example. Since we are using an oxygen sensor with a heater, you need to first measure the resistance for the internal heater.

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  • Turn on your multimeter
  • Put the setting on Ohms (Ω) and set it to 200
  • Test the o2 sensor response to a lean fuel consumption situation. Then, disconnect the hose from the positive crankcase ventilation (PVC) valve which is located on the valve cover. This will allow more air into the engine so the voltmeter should read close to 200mV (0.20V). If the voltmeter does not respond, the o2 sensor is not functioning properly.
  • Reconnect the PVC hose to test the o2 sensor’s response to a rich fuel consumption situation. In order to do this, disconnect the plastic hose connection to the air cleaner assembly. Block the hose connection opening with a rag in order to reduce the amount of air going into the engine.
  • Check the voltmeter. It should read close to 800mV (0.08V) due to the reduction of oxygen entering the engine. If the o2 sensor does not respond this way, it is not functioning properly.
  • Reconnect the hose to the air cleaner.
  • If the o2 sensor responded correctly to the lean and rich fuel tests, another component could be causing the oxygen sensor code problem. The potential issues could be a vacuum leak, ignition system or something similar. Obviously, if the o2 sensor did not respond properly then it is bad and will need to be replaced.

Conclusion

So, after all your testing, you should know whether the o2 sensor is bad or if something else is the problem.

In the event that you feel confident that the o2 sensor is bad, you can tackle it yourself. If you’re not sure that the o2 sensor is bad, you should probably take your car to a professional. Remember, addressing the problem sooner than later may save you from more serious issues such as replacement of the catalytic converter

See also: TPS Sensor Symptoms

Read: Bad MAF Sensor?

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