Open or short circuit to earth in the heater control wiring for Bank 1 Sensor 3; Faulty heater element inside the oxygen sensor (internal short or high current draw); Damaged, corroded or loose connector at the sensor or in the harness; Blown fuse or fault in the heater power supply or relay, where fitted; Exhaust heat or road damage to the sensor wiring, such as chafing or melted insulation; Fault in the engine control unit heater driver circuit.

P0043

HO2S Heater Control Circuit Low 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
43: FaultThe specific fault in that area

What it means

P0043 means the engine control unit has detected a low voltage or low signal state in the heater control circuit of the heated oxygen sensor at Bank 1, Sensor 3. This is the third sensor on the Bank 1 side, usually located further downstream of the catalytic converter. The control unit monitors the heater circuit and set the code because the voltage or current did not match what it expected when it commanded the heater on or off.

Symptoms

Malfunction indicator lamp on
Usually no noticeable change in driveability
Slightly slower sensor readiness, which may cause an emissions or readiness monitor to stay incomplete
Possible failed emissions test or MOT readiness issue
Other oxygen sensor heater codes stored alongside it

Possible causes

Open or short circuit to earth in the heater control wiring for Bank 1 Sensor 3likely
Faulty heater element inside the oxygen sensor (internal short or high current draw)likely
Damaged, corroded or loose connector at the sensor or in the harnesslikely
Blown fuse or fault in the heater power supply or relay, where fittedpossible
Exhaust heat or road damage to the sensor wiring, such as chafing or melted insulationpossible
Fault in the engine control unit heater driver circuitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data to see the conditions when the code set
Confirm the sensor numbering for this specific model and engine, as Bank 1 Sensor 3 is not fitted to every vehicle
Visually inspect the sensor wiring and connector for chafing, heat damage, corrosion, water ingress and poor terminal tension
Check the relevant fuse and relay for the heater supply, using the wiring diagram for the vehicle
With the ignition on, check for correct supply voltage at the sensor heater feed and a good earth or control signal as per the manufacturer data
Measure the heater element resistance at the sensor and compare it with the specification for that sensor
Check the heater control wire for continuity and for a short to earth between the sensor connector and the control unit
Clear the code and road test, then recheck that the code does not return and the monitors complete

How to fix

Repair or replace damaged wiring and connectors in the heater circuit
Replace the oxygen sensor if the heater element is out of specification
Replace a blown fuse or faulty relay once the cause of the failure has been found
Secure and reroute any wiring that is close to hot exhaust parts or likely to chafe
Repair or replace the control unit only after all wiring and the sensor have been proven good

Can I keep driving?

Low severity

The car can normally be driven as the fault usually has little effect on how it runs. Book it in soon to keep emissions compliant and to avoid the fault masking other sensor problems.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.