Low voltage on upstream O2 sensor heater bank 1.
P0031 — also written POO31, P 0031, 0031 — is the OBD-II code for ho2s heater control low (bank 1 sensor 1). Same code, same fix: this page covers every spelling of it.
O2 sensor heater circuit low (bank 1 sensor 1) — the pre-cat lambda's built-in heater element is drawing too little current, so the ECU can't warm it up fast enough for closed-loop fuelling.
P0031 is a sensors fault code, typically costing £80–£300 to put right in a UK garage.
What P0031 actually feels like to drive
- MPG drop of 4–8 (engine stays in open-loop / cold-start fuelling too long)
- Slight sooty smell from exhaust when cold
- MOT emissions borderline pass or fail on the CO/HC readings
- Engine light on but no driveability complaint
- Code returns immediately after clearing (heater is either working or it isn't)
What causes P0031?
- Sensor heater failure
- Open circuit
- Bad ground
UK vehicles that throw P0031 most often
- Vauxhall Corsa / Astra 1.2/1.4 (A12XER, A14XER) — sensor heater failure at 60–90k
- Ford Fiesta / Focus 1.25 / 1.6 (Sigma / Duratec) — heater wire chafed on exhaust
- Nissan Micra / Note 1.2 (HR12DE) — genuine sensor internal failure
- VW Polo / Golf 1.2 / 1.4 TSI — heater fuse/relay in engine bay fusebox
- Toyota Yaris / Aygo 1.0 / 1.33 — corroded sensor connector
How P0031 is fixed
- Replace sensor
- Wiring repair
The P0031 misdiagnosis that wastes money
Most P0031s are genuinely a failed sensor — but not before you check the connector and the fuse. On Ford Fiestas and Focuses we routinely see the heater feed wire chafed against the exhaust manifold heat shield; a £4 length of high-temperature loom tape and 20 minutes of repair fixes it and no new sensor is needed. On VAG 1.2/1.4 TSI, there's a dedicated O2-heater fuse in the engine-bay fusebox that blows if the heater short-circuits; replacing just the fuse is a false economy because the sensor is the reason it blew, but always check that fuse first so you know what you're dealing with. Genuine Bosch/Denso/NTK sensors last 100k+ — cheap eBay sensors at £25 fail within a year and store P0031 again.
What a UK garage should charge for P0031
UK independent garage: diagnostic + circuit test £30–£60. Pre-cat lambda sensor replacement £110–£220 fitted (genuine or OE-supplier). Connector or wiring repair £40–£90. Fusebox / fuse replacement £15–£40 if that's all it needs. Main dealer: £180–£340 for the sensor. Always specify Bosch, Denso or NTK — never generic — for anything used past its emissions test.
P0031 repair cost in the UK
Most P0031 repairs land between £80–£300 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0031
Before authorising a new sensor for P0031, ask the garage to check for a chafed heater wire near the exhaust manifold heat shield — 5 minutes' work and it's the actual fault on maybe a quarter of these cars. If a new sensor is genuinely needed, fit an OE-branded part (Bosch, Denso or NTK) not an unbranded box from eBay; a £30 sensor is not saving you money if it fails the MOT emissions test in 12 months. And on any car where P0031 keeps returning after a new sensor, get the exhaust manifold gasket checked — a small pre-cat leak lets in cold air, the sensor takes longer to warm, and the ECU flags the heater as underperforming.
Can you drive with P0031?
Drivable.
Urgency: low.