P0118–P2185 Engine Coolant Temperature Sensor 1 Circuit High

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

P0118: Engine Coolant Temperature Sensor 1 Circuit High.

P0118 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

1. Unplug the sensor and check whether the signal falls. If it stays high, suspect the wiring rather than the sensor.
2. Check the earth path - a missing earth makes a sensor read high.

What actually causes this on a VAG

This is the engine's own coolant temperature sensor - the one the ECU uses to decide how much fuel to inject, which on many VAG engines is a second element inside the same plastic sensor that feeds the dash gauge. "Circuit high" means the ECU is seeing the voltage that corresponds to an impossibly cold engine, so it points at an open circuit or a dead sensor rather than an actual cold engine. Typical symptoms are rich running, a rough or hunting warm idle, worse fuel economy, a temperature gauge that sits low or behaves oddly, and - on cars where the ECU controls the cooling fans - fans that run more than they should.

Most likely causes, in order

1. The coolant temperature sensor itself. These plastic sensors are a well-known wear item right across the VAG range and often fail intermittently - reading correctly from cold, then dropping out once everything is hot. Confirm it with live data: if the reported coolant temperature is wildly cold on a hot engine, or flicks to the very cold end of the scale as it warms up, the sensor is the prime suspect.
2. Connector or wiring at the sensor. The plug sits in a hot part of the engine bay and both the contacts and the little locking clip go brittle with age, so a spread or corroded terminal gives exactly the same open-circuit reading as a dead sensor. With live data on screen, gently wiggling the plug and the last few inches of loom will usually make the reading jump if this is it.
3. The plastic housing the sensor sits in. On many VAG engines the sensor pushes into a plastic coolant flange and is held by a metal spring clip rather than being screwed in. The flange cracks and the clip weakens, letting the sensor sit proud, weep coolant and lose good contact - look for dried coolant crust or staining around the sensor and the flange seam.
4. A shared earth or loom fault on the ECU side. Less common, but a poor sensor earth produces the same symptom as a broken sensor. If other unrelated sensor codes stored at the same moment, look at the shared earth point and loom rather than buying a coolant sensor.

Worth knowing: The sensor is usually under coolant pressure and held in only by a spring clip, so releasing that clip on a hot engine can fire it out along with scalding coolant. Let the engine go properly cold first, and if the plastic housing it sits in is already cracked or crusty with dried coolant, replace the housing and seal at the same time rather than doing the job twice.

Common questions

What does P0118 mean?

P0118 means Engine Coolant Temperature Sensor 1 Circuit High. The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.

Can I drive with P0118?

This class of fault can cause real mechanical damage if you keep driving on it. A persistent misfire also destroys the catalytic converter, which is the expensive part.

Related reading

• VW or Audi Overheating: What to Do Now and What Causes It
• Losing coolant with no visible leak on a VW or Audi
• EA888 1.8 and 2.0 TSI Problems: An Owner’s Guide
• Why the same fault code has completely different causes