Failed heater element inside the bank 1 sensor 1 oxygen sensor (internal short or low resistance); Heater control wire chafed and shorted to earth, or damaged insulation near the exhaust or engine; Corroded, damaged or loose connector at the oxygen sensor; Blown fuse or faulty relay supplying the heater circuit (depends on model and circuit design); Poor earth or low supply voltage affecting the heater circuit; Faulty heater driver inside the engine control unit.

P0031

HO2S Heater Control Circuit Low Bank 1 Sensor 1.

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
31: FaultThe specific fault in that area

What it means

P0031 means the engine control unit has detected a low voltage or low signal state in the heater control circuit of the heated oxygen sensor on bank 1, sensor 1, which is upstream of the catalytic converter. The control unit switches the heater on and off and monitors the circuit, and it set the code because the circuit voltage or current was lower than expected. This is usually caused by a short to earth, a faulty heater element or a wiring or driver fault.

Symptoms

Engine warning light on
Poor fuel economy
Rough running or hesitation, especially on a cold start
Slow or erratic closed loop fuel control after starting
Failed emissions test
Possible rich running smell or black exhaust smoke

Possible causes

Failed heater element inside the bank 1 sensor 1 oxygen sensor (internal short or low resistance)likely
Heater control wire chafed and shorted to earth, or damaged insulation near the exhaust or enginelikely
Corroded, damaged or loose connector at the oxygen sensorpossible
Blown fuse or faulty relay supplying the heater circuit (depends on model and circuit design)possible
Poor earth or low supply voltage affecting the heater circuitpossible
Faulty heater driver inside the engine control unitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including engine temperature and run time since start
Check for related fault codes such as other oxygen sensor heater or circuit codes, and check battery voltage
Visually inspect the sensor wiring and connector for chafing, melting, oil contamination, corrosion or poor pin tension
Check the relevant fuse and relay for the heater supply, using the wiring diagram for the model
Measure the heater element resistance at the sensor connector and compare it with the manufacturer specification
Check the heater control wire for a short to earth and for continuity back to the control unit with the ignition off and the connector disconnected
Check the heater supply voltage and earth at the connector with the ignition on, and watch the control signal operate with a suitable scan or scope
Clear the code and road test to confirm whether it returns after the repair

How to fix

Replace the bank 1 sensor 1 oxygen sensor if the heater element is out of specification
Repair or replace damaged wiring that is shorted to earth, and secure it away from heat and sharp edges
Clean or replace a corroded or damaged connector and terminals
Replace a blown fuse only after finding the cause of the overload, or replace a faulty relay
Repair poor earth or supply faults in the heater circuit
Diagnose and, if proven faulty, repair or replace the control unit after all wiring and sensor faults are ruled out

Can I keep driving?

Low severity

The car can normally be driven in the short term, but book it in soon because fuel economy and emissions may suffer. If the engine runs badly or the fault is accompanied by other warnings, have it checked sooner.

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