Powertrain, generic · Fuel and air metering, auxiliary emission controls

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

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

The upstream oxygen sensor on bank 1 (sensor 1, before the catalytic converter) only delivers a usable signal once its ceramic element is hot. A heating element inside the sensor gets it there quickly after a cold start. Typically the heater receives battery voltage via a fuse or relay, and the engine control module switches the ground side with a pulsed signal while comparing the commanded state with the measured voltage or current. P0031 means the control module measures too little voltage or current while the heater is switched on. Depending on the system this points to a burnt-out element, a missing supply or an interrupted control wire. The sensor then warms up late, so mixture control starts late and cold-start emissions rise.

Likely causes

Heating element burnt out or high resistance (too little current)
Supply wire interrupted or fuse blown
Control wire to the engine control module interrupted
Corroded connector with high contact resistance
Output driver in the engine control module faulty

Symptoms

  • Check engine light on
  • Closed-loop control starts late after a cold start
  • Further heater codes if a shared fuse has blown
  • Emissions test or readiness check failed

Repair estimate

DifficultyMediumDIY possible
Parts and labour€80–350
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

oxygen sensor upstreamoxygen sensor heateroxygen sensor wiringoxygen sensor heater fuse

Related codes

P0030P0032P0037P0051P0053P0135

References: law.cornell.edu, saabwisonline.com, saabwisonline.com, en.wikipedia.org.