Failed internal heater element in the Bank 1 Sensor 3 oxygen sensor; Damaged, chafed or corroded wiring to the sensor heater, including short or open circuit; Poor connection or corroded terminals at the sensor plug; Blown fuse or faulty relay in the heater power supply; Exhaust heat or water damage to the sensor harness near the exhaust; Fault in the control unit's heater driver circuit.

P0042

HO2S Heater Control Circuit 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
42: FaultThe specific fault in that area

What it means

P0042 is the generic code for a fault in the heater control circuit of the heated oxygen sensor on Bank 1, Sensor 3. Sensor 3 is normally the third oxygen sensor along the exhaust, fitted downstream of the catalytic converter, often after a second catalyst or further back on the exhaust. The control unit sets the code when the heater circuit's current, voltage or resistance falls outside the expected range, or when the sensor does not warm up as it should.

Symptoms

Engine warning light on
Little or no change in driveability in most cases
Possible failed emissions test
Other oxygen sensor or catalyst monitor codes stored alongside
Readiness monitors for the oxygen sensor or catalyst not completing

Possible causes

Failed internal heater element in the Bank 1 Sensor 3 oxygen sensorlikely
Damaged, chafed or corroded wiring to the sensor heater, including short or open circuitlikely
Poor connection or corroded terminals at the sensor plugpossible
Blown fuse or faulty relay in the heater power supplypossible
Exhaust heat or water damage to the sensor harness near the exhaustpossible
Fault in the control unit's heater driver circuitrare

How to diagnose

Read all stored codes and freeze frame data, and note any other oxygen sensor, catalyst or fuse-related codes.
Confirm which sensor is Bank 1 Sensor 3 on this model, as exhaust layout varies by engine and market.
Visually inspect the sensor wiring and plug for melting, chafing, corrosion or exhaust contact.
Check the fuse and relay supplying the oxygen sensor heater, using the wiring diagram for the vehicle.
Measure the heater element resistance at the sensor connector and compare it with the manufacturer's specification.
With the plug disconnected, check for battery supply and a good earth or control signal at the harness side as the wiring diagram shows.
Check the heater circuit wiring for continuity and for shorts to earth or supply between the sensor and the control unit.
After repair, clear the code and run a drive cycle to confirm the heater circuit passes its monitor.

How to fix

Replace the Bank 1 Sensor 3 oxygen sensor if the heater element is open or out of specification.
Repair or replace damaged wiring and secure it away from hot or sharp exhaust parts.
Clean or replace corroded connector terminals or the sensor plug.
Replace the blown fuse after finding why it failed, or replace a faulty relay.
Repair or replace the control unit only after wiring, supply and the sensor have been proven good.

Can I keep driving?

Low severity

The car can normally be driven as usual while you book a repair, as this sensor sits after the catalyst and rarely affects running. Get it fixed in good time, as it may cause an emissions test failure and mask other faults.

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