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P0166 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


A problem with the O2 sensor circuit for Bank 2 Sensor 3 of your vehicle is reported in DTC code P0166. For some vehicles, this code is only applicable to them and may not apply to other vehicle types or models. In order to ensure a correct interpretation of this Code, it is essential that the vehicle’s documentation or an automotive expert be consulted.


The P0166 code indicates that the O2 sensor in Bank 2 Sensor 3 is sending abnormal or erratic voltage signals to the vehicle’s engine control module (ECM) or powertrain control module (PCM). The ECM/PCM relies on data from the O2 sensor to adjust the air-fuel mixture, and a malfunctioning sensor can adversely affect the engine’s performance and emissions.


The P0166 DTC code might be caused by a number of things:

  • Faulty O2 Sensor: If the O2 sensor is faulty, it might provide erroneous readings and set off the error code.
  • Wire Problems: Corroded or damaged wire in the O2 sensor circuit might prevent the signal from being sent, which will result in the code.
  • Exhaust System Leaks: The O2 sensor’s capacity to deliver reliable readings might be hampered by exhaust system leaks that pollute it.
  • ECM/PCM Issues: The O2 sensor may give false readings if there is an issue with the vehicle’s engine control module or powertrain control module.


There are a number of signs that might be seen when the P0166 code is set off:

  • Check Engine Light: The most noticeable symptom is the illumination of the check engine light on the vehicle’s dashboard.
  • Reduced Fuel efficiency: An O2 sensor issue might make the engine perform less effectively, which lowers fuel efficiency.
  • Poor Engine Performance: The car might have rough acceleration, a hesitant start, or harsh idling.
  • Emission Problems: Due to higher emissions brought on by an incorrect air-fuel combination, the vehicle may fail an emissions test.


A skilled technician must use a methodical strategy to diagnose the P0166 DTC code. The diagnostic procedure might entail:

  • OBD-II Scanner: Using an OBD-II scanner, the mechanic can retrieve the trouble code and other relevant data from the vehicle’s onboard computer.
  • Visual Inspection: The O2 sensor and its wires are visually inspected for any obvious deterioration or damage.
  • Testing the O2 Sensor: The mechanic may do experiments to examine the voltage output and reaction to modifications in the exhaust gas composition of the O2 sensor.
  • Handling Related Issues: Prior to doing the O2 sensor examination, the mechanic should take care of any exhaust leaks or other relevant issues.

Common mistakes

Several typical errors with the P0166 code should be avoided, including:

  • Ignoring Other Codes: P0166 may occasionally be found alongside other related codes. Ignoring them could result in omitting important concerns.
  • Replacing the Wrong Component: An incorrect diagnosis may result in replacing an O2 sensor that is still operational rather than addressing wiring or ECM/PCM problems.
  • Ignoring Maintenance: The performance of the O2 sensor and other engine parts might be affected by routine maintenance, such as cleaning or changing air filters.

How serious is this?

The seriousness of the P0166 DTC shall be determined by a number of factors, in particular vehicle make and model, operating conditions, and the extent to which this problem is present. While it may still be possible to drive with the code, disregarding this can result in decreased fuel efficiency and potential damage to the catalytic converter. In order to avoid further complications, it is advisable to resolve the problem as soon as possible.

What repairs can fix the code?

repair manuals

The following fixes can resolve the P0166 DTC code, according to the diagnosis:

  • Changing the O2 Sensor: If the O2 sensor is broken, changing it should fix the problem.
  • Repairing wire: The O2 sensor circuit’s broken or corroded wire can be fixed to enable correct signal transmission.
  • Sealing Exhaust Leaks: Sealing leaks in the exhaust system will stop the O2 sensor from being further contaminated.
  • ECM/PCM Reprogramming or Replacement: If the engine control module or powertrain control module is at fault, reprogramming or replacing it may be necessary.

Related codes

Other related DTC codes include P0167 (O2 Sensor Circuit Low Voltage Bank 2 Sensor 3) and P0168 (O2 Sensor Circuit High Voltage Bank 2 Sensor 3). These codes may provide additional insights into the underlying issue.


For Bank 2 Sensor 3, a DTC code of P0166 could be an early indication of possible problems with O2 sensors. In order to ensure optimum engine performance, fuel efficiency, and compliance with emissions standards, timely diagnosis and appropriate repair are essential. If this code has been detected, it is recommended that you seek professional automotive assistance to correctly diagnose and fix the problem so as to improve your driving experience without any problems.

P0166 Code – What Does It Mean & How To Fix It