CodeXOBD

P0031 — HO2S Heater Control Circuit Low (Bank 1 Sensor 1)

P0031 means the oxygen sensor heater is drawing less current than expected, usually because the heater element is open or the wiring has high resistance.

Low

The engine runs normally once warm but cold-start fuel control is impaired.

$20–$200Parts, typical range
0.3–1 hLabour
estimateNot a quote · 2026 basis

What P0031 means

P0031 is a more specific variant of P0030. Where P0030 reports a general circuit fault, P0031 specifically indicates low current — meaning the heater element is likely open-circuit or the wiring has excessive resistance. The effect is the same: the sensor cannot heat itself and relies on exhaust gas temperature alone.

Technical summary
SystemEmission control
How the catalytic converter is gradedExhaust leaves the engine, passes the upstream oxygen sensor, then the catalytic converter, then the downstream oxygen sensor. The module compares the two sensor signals: on a healthy converter the downstream signal is steady while the upstream one oscillates. When the downstream signal starts to mirror the upstream one, the converter is judged to have stopped storing oxygen.Engineexhaust outUpstream O₂sets fuel trimCatalytic converterstores oxygenDownstream O₂grades the converterThe module compares the two sensors — that comparison is the test

Symptoms you may notice

What causes P0031

Heater element open-circuit

Very likelylow costeasy

The most common cause — the internal heater wire has broken, creating an open circuit that draws no current.

Corroded connector or damaged wiring

Likelylow costeasy

Corrosion or damage in the connector or harness adds resistance, reducing current below the threshold the module expects.

Blown O2 heater fuse

Possiblelow costeasy

A shorted sensor heater can blow the dedicated fuse, which then prevents current from flowing on subsequent starts.

How to diagnose it

1

Measure heater resistance

Disconnect the sensor and measure resistance across the heater terminals. An open reading confirms a failed heater.

Tools: MultimeterExpected: 4-40 ohmsIf it passes: Resistance is normal — check wiring and connector for corrosion.If it fails: Open circuit — replace the sensor.
2

Check the heater fuse

Inspect the fuse for the upstream O2 heater circuit. If it is blown, the heater may have shorted and blown the fuse as a protection measure.

Tools: Fuse pullerIf it passes: Fuse is intact. Continue to step 3.If it fails: Replace the fuse. If it blows again, the sensor or wiring is shorted.
3

Check voltage supply to the heater

With the sensor disconnected, turn the ignition on and measure voltage at the heater power wire in the harness connector. It should read battery voltage.

Tools: MultimeterIf it passes: Battery voltage present. The sensor is confirmed faulty.If it fails: No voltage — trace the circuit back to the fuse and relay for an open.

Related codes

P0030P0032

Common questions

What is the difference between P0030 and P0031?

P0030 is a general heater circuit fault. P0031 specifically means low current — the heater is likely open-circuit. P0032 means high current, suggesting a short.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.