EngineLightCodes
Powertrain code

P0118 Engine Coolant Temperature Sensor 1 Circuit High

Severity
Fix soon
Typical repair cost
$20–$160
Do it yourself
Possible, easy
Shop time
0.3–1.5 hours

The engine coolant temperature sensor 1 is an NTC thermistor: the control module feeds 5 V through a resistor, and as the coolant warms the resistance and signal voltage fall.

How the engine computer detects it

Its value drives cold-start enrichment, idle speed, ignition timing, the radiator fan and the release of closed-loop mixture control. Some Ford engines measure cylinder head temperature instead and calculate a coolant value from it. P0118 means the signal voltage sits near the 5 V reference, so the sensor would report about −40 °C (−40 °F). Typical causes are an unplugged or corroded connector, a broken signal or sensor ground wire, an open sensor or a short to voltage. Limits: GM above 134 kilohms (only after 10 s running, so genuine cold is not misread), Ford gasoline above 4.96 V for 5 s, Ford diesel above 4.91 V, GM/Saab below −39 °C (−38 °F) for 4 s. Without a substitute strategy the mixture would be far too rich; GM/Saab run the fan and switch off the A/C. On Ford engines with a head temperature sensor the equivalent is P1289.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

Symptoms

  • Check engine light on
  • Radiator fan runs constantly
  • Temperature gauge stays cold
  • Rich mixture, higher consumption, possibly black smoke or fuel smell
  • Raised idle speed

Common causes

  1. Connector unplugged or corroded, contact open Most likely
  2. Broken signal or sensor ground wire Possible
  3. Sensor open internally Possible
  4. Signal wire shorted to 5 V or battery voltage Less likely
  5. Engine control module faulty Less likely

How to diagnose P0118

  1. Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
  2. Inspect the parts involved: engine coolant temperature sensor, engine coolant temperature sensor wiring.
  3. Work through the causes above from most to least likely, cheapest checks first.
  4. Clear the code and drive a full drive cycle to confirm the repair held.

Reading the code

P
Powertrain
0
Generic (SAE) code
1
Fuel and air metering
18
Specific fault

Questions drivers ask

Can I drive with code P0118?

Drive gently and only as far as you need to. The engine computer may limit power to protect the engine, so avoid highways and heavy traffic until it's fixed. If the check engine light starts flashing, pull over.

Will my car pass an emissions inspection with P0118?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0118 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0118 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.