P0166 Code Definition
O2 Sensor Circuit No Activity Detected (Bank 2, Sensor 3)
What the P0166 code means
The P0166 code means that the third oxygen sensor on the second bank of the exhaust has failed or is not responsive.
The powertrain control module is not receiving a signal from this particular oxygen sensor. This may also be due to the sensor being unplugged.
What are the causes of the P0166 code?
- An unplugged oxygen sensor
- Faulty oxygen sensor
- Damaged electrical connector or wiring associated with the oxygen sensor
- Faulty powertrain control module
What are the symptoms of the P0166 code?
Symptoms that a driver may experience with a P0166 would include decreased fuel mileage, incorrect air/ ratio which may cause the engine to run incorrectly, and of course the Check Engine Light being illuminated.
How does a mechanic diagnose the P0166 code?
A mechanic may diagnose the P0166 code by using an OBD-II scanner to run data on the vehicle while the engine is running.
Assuming the scan tool has the capability, the signal voltages given from the oxygen sensors should be displayed while running data if the oxygen sensors are working correctly.
If no reading is given from any of the oxygen sensors, the sensor in question may be unplugged or faulty. The mechanic would then give the oxygen sensors a visual inspection to make sure that they are all plugged in.
If the sensors are all plugged in and none of the wiring has been damaged, then the mechanic may test the one in question for functionality with the manufacturer’s specific procedure. If all of these tests have been performed and the fault has not been found yet, the issue likely lies within the powertrain control module.
Common mistakes when diagnosing the P0166 code
A common mistake that may be made while diagnosing the P0166 code would be replacing the oxygen sensor without properly testing it. The manufacturer posts testing procedures for common faults for a reason.
Any code that can be stored in a vehicle’s powertrain control module likely has a diagnostic procedure provided by the manufacturer. It is best to follow these procedures to avoid misdiagnosis and save time and money.
How serious is the P0166 code?
This code is not very serious as long as it is not associated with any other trouble codes. Especially considering that this code is related to the third oxygen sensor on the second bank of the exhaust.
This means that the engine is likely a V-shaped engine that has at least six oxygen sensors. With the loss of one sensor, the powertrain control module should be able to maintain a correct air/fuel ratio with the inputs from all of the other sensors.
What repairs can fix the P0166 code?
- Plugging the oxygen sensor back in
- Replacing the oxygen sensor
- Repairing or replacing wiring that is associated with the oxygen sensor.
- Replacing the powertrain control module
Additional comments regarding the P0166 code
It is not a good idea to drive your vehicle for an extended period of time with a Check Engine Light. With this in mind it is a good idea to have this code diagnosed and repaired as soon as possible although this particular code is not as serious as some other trouble codes may be.
Need help with a P0166 code?
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This article originally appeared on YourMechanic.com as P0166 OBD-II Trouble Code: O2 Sensor Circuit No Activity Detected (Bank 2, Sensor 3) and was authored by Andrew Quinn.