Coolant temperature sensor reading open-circuit on Audi — usually the sensor or its brittle plug, occasionally a thermostat housing leak soaking the connector.
Audi P0118 — Coolant temperature sensor reading open-circuit on Audi — usually the sensor or its brittle plug, occasionally a thermostat housing leak soaking the connector. Typical UK independent garage cost: £40–£260.
What actually causes P0118 on a Audi
On EA888 petrol engines the coolant temperature sensor sits in the plastic thermostat housing on the side of the block. Those housings are known to weep, and coolant tracks down into the two-pin sensor plug and corrodes it. The ECU then sees an open circuit, reads the engine as extremely cold and sets P0118. On the TDI engines the sensor is in the coolant flange at the back of the head. The plug clip goes brittle with heat and cracks when someone's been in there before, so the connector backs off slightly. It's rarely the ECU. Around a third of the time it's the plug or wiring rather than the sensor itself, which is why the cheap check comes first. The giveaway on an Audi is that the radiator fans run on full as soon as you start it from cold, and the dash temperature gauge sits at the bottom or jumps about.
Audi engines that log P0118
- 1.4 / 2.0 TFSI (A3, A4, A5, Q3, Q5)
- 1.8 TFSI EA888 (A4 B8, A5)
- 2.0 TDI EA189 / EA288 (A3, A4, A6)
- 3.0 TDI V6 (A6, Q7)
How we diagnose P0118 on a Audi
- Read live data: coolant temperature showing around -40°C with a warm engine means an open circuit, so it's wiring or sensor
- Unplug the sensor and inspect the pins for green corrosion or coolant inside the connector body
- Check the thermostat housing (EA888) or coolant flange (TDI) for a coolant weep above the plug
- Measure sensor resistance: roughly 2–3 kΩ cold and a few hundred ohms hot. Open circuit means a new sensor
- Fit a genuine or OE-supplier sensor (it has a new O-ring and clip), bleed the cooling system with G13 coolant and clear the code
The common misdiagnosis to avoid
Replacing the thermostat because the gauge reads cold. A stuck-open thermostat gives a slow warm-up and often P0128, but it doesn't make the sensor read open-circuit. P0118 is an electrical fault: check the sensor and plug first. The other trap on EA888 is fitting a new sensor into a leaking thermostat housing. The new plug corrodes within months and the code comes back.
Audi P0118 repair cost in the UK
Expect £40–£260 including parts and labour at an independent garage. Main-dealer quotes on Audi are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.
Bob's workshop tip
Audi EA888 with P0118: look up at the thermostat housing before you buy anything. If it's damp with pink crust, fix the leak at the same time or you'll be doing the sensor twice.
Codes usually seen alongside P0118 on Audi
- P0117
- P0116
- P0128
- P0125
Symptoms drivers report with P0118
- The cooling fans running at full blast even when you've just started the car from cold.
- The car is a nightmare to start in the morning, chugging like an old tractor.
- The temp gauge on the dash is completely dead or jumping erratically.
- Black smoke out the back and a strong smell of unburnt petrol/diesel.
- The stop-start system has completely stopped working for no apparent reason.
Repairs that clear P0118
- Replace ECT sensor
- Repair wiring
For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0118 guide.
Reading P0118 on a Audi properly
A generic reader will show P0118 and nothing else. What matters is the freeze-frame data stored with it — engine speed, load, coolant temperature and fuel trims at the moment the fault was logged — plus whether any other module on the Audi has logged something at the same time. Codes rarely arrive alone, and the order they were stored in usually points at which one is the cause and which are the consequences.
Clearing P0118 without fixing it
Erasing the code resets the Audi's readiness monitors as well as the light. The fault returns on the next drive cycle if the cause is still there, and in the meantime the incomplete monitors can stop the car passing an emissions check. After a genuine repair, the car is driven until the relevant monitor completes before it is handed back.
Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0118 is showing on your Audi, bring any scan results with you — we will read the live data ourselves, confirm which cause it actually is, and price the repair before any work starts.