P0126
Insufficient Coolant Temperature for Stable Operation
- Severity
- Medium
- Driving
- Drivable — book it in
The engine is not reaching normal operating temperature quickly enough for the module to run the engine on its normal warmed-up settings. Until it is warm the module runs a deliberately rich cold-start mixture and ignores the oxygen sensors, so an engine that never warms up properly runs rich permanently. By far the most common cause is a thermostat stuck open, and that is a cheap part.
Possible causes
- CommonThermostat stuck open, so the coolant never reaches operating temperature
- CommonCoolant temperature sensor reading lower than the truth
- PossibleThermostat missing, or one of the wrong temperature rating fitted
- PossibleLow coolant level
- PossibleWiring or connector fault at the coolant temperature sensor
- PossibleGenuinely extreme cold combined with a lot of short journeys
What you might notice
- Poor fuel economy - often noticeably worse than usual
- Weak or lukewarm cabin heating
- Temperature gauge slow to rise, or never reaching the middle
- Failed emissions test
- Check engine light on
What to check, in order
- Notice whether the cabin heater blows properly hot. Weak heat plus slow warm-up is the classic thermostat-stuck-open picture, and it costs nothing to check on the drive in.
- Watch the coolant temperature on live data through a warm-up. It should climb steadily and settle around the thermostat's rating; one that creeps up and stalls short of it, or falls again at motorway speed, is a stuck-open thermostat.
- Check the sensor before the thermostat if the gauge and the live data disagree. A sensor reading low makes a healthy engine look cold to the module.
- Check the coolant level. Low coolant can leave the sensor reading air rather than liquid.
- Do not ignore this because the car drives normally. It runs rich the whole time, which costs fuel, dilutes the oil with unburnt petrol and shortens the catalytic converter's life.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Fuel and air metering
P0100P0101P0102P0103P0104P0105P0106P0107P0108P0109P0110P0111P0112P0113P0114P0115P0116P0117P0118P0119P0120P0121P0122P0123P0124P0125P0126P0127P0128P0129P0130P0131P0132P0133P0134P0135P0136P0137P0138P0139P0140P0141P0142P0143P0144P0145P0146P0147P0148P0149P0150P0151P0152P0153P0154P0155P0156P0157P0158P0159P0160P0161P0162P0163P0164P0165P0166P0167P0168P0169P0170P0171P0172P0173P0174P0175P0176P0177P0178P0179P0180P0181P0182P0183P0184P0185P0186P0187P0188P0189P0190P0191P0192P0193P0194P0195P0196P0197P0198P0199