P0055
HO2S Heater Resistance Bank 1 Sensor 3.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 0
- AreaFuel and air metering, auxiliary emission controls
- 55
- FaultThe specific fault in that area
What it means
P0055 means the engine control unit has found the heater circuit resistance of the heated oxygen sensor at Bank 1, Sensor 3 to be outside its expected range. Sensor 3 is the third oxygen sensor on Bank 1, normally fitted further downstream of the catalytic converter, and its position depends on the exhaust layout. The control unit measures the heater's resistance or current and sets the code when the reading is too high or too low.
Symptoms
- Engine management light on
- Usually no noticeable change in driving
- Possible slight increase in fuel consumption on some models
- Failed emissions or MOT test
- Other oxygen sensor heater codes stored alongside it
Possible causes
- Failed internal heater element in the Bank 1 Sensor 3 oxygen sensorlikely
- Damaged, chafed or melted wiring to the sensor heater, often near the exhaustlikely
- Corroded or loose sensor connector with high resistance in the heater circuitlikely
- Blown fuse or faulty heater power supply or relay, where fittedpossible
- Wrong or poor quality replacement sensor with the wrong heater resistancepossible
- Fault in the engine control unit heater driverrare
How to diagnose
- Read all stored codes and freeze frame data, and note whether other oxygen sensor heater codes are present
- Confirm which sensor is Bank 1 Sensor 3 on this engine, using the model-specific exhaust layout
- Visually inspect the sensor wiring and connector for heat damage, chafing, water ingress and corrosion
- Check the relevant fuse and any heater supply relay against the wiring diagram
- Measure the sensor heater resistance across its heater terminals and compare with the manufacturer's specification
- Check for battery supply and a good earth or control signal at the connector with the ignition on
- Check the wiring for continuity and short circuits between the sensor connector and the control unit
- Check that the fitted sensor is the correct type for the vehicle
How to fix
- Replace the Bank 1 Sensor 3 oxygen sensor if the heater resistance is out of specification
- Repair or replace damaged wiring and secure it away from hot or moving parts
- Clean or replace the corroded or damaged connector terminals
- Replace the blown fuse or faulty relay after finding why it failed
- Fit the correct specification sensor if the wrong part was used
- Diagnose and repair or replace the control unit only after all wiring and sensor checks pass
Can I keep driving?
Low severity
The car can normally be driven as it is, as this fault rarely affects how the engine runs. Book it in soon, as it will cause an MOT emissions failure and may hide other faults.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.