Number P0118: Engine Coolant Temperature Sensor 1 Circuit High
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. Its value drives cold-start enrichment, idle speed, ignition timing, the radiator fan and the release of closed-loop mixture
Can you drive with P0118?
Severity: Fix promptly — drivability fault. Short trips at moderate speed only. If the car has entered limp mode (won't rev, stuck in one gear), have it towed rather than forcing it.
If you ignore it: Limp-mode faults tend to strand you, and several of them (misfires, throttle faults) damage other parts while you wait.
This is the #40 most common code in our top-150 library, in the cooling system family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0118 means
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. 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. 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 petrol above 4.96 V for 5 s, Ford diesel above 4.91 V, GM/Saab below −39 °C 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.
Systems involved: engine coolant temperature sensor, engine coolant temperature sensor wiring.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 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 common |
| 5 | Engine control module faulty | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- 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
Misdiagnosis traps
- A code that appears after coolant work is often an air pocket around the sensor or the connector left off — the sensor reads steam, not coolant.
- A reading pinned at −40° or maximum usually means wiring, not a sensor that failed to exactly those extremes.
How a good shop confirms it
- Verify actual coolant temperature with an infrared thermometer at the housing and compare to the scan value.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Coolant temperature sensor | $58 – $170 | $20 – $70 (parts only) | 1/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor).
OBDex repair data for this code rates the job difficulty easy with a typical labor time in the 0.3–1.5 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
Why does the temperature gauge act strange too?
On many cars the same sensor feeds the gauge. A fault can peg the gauge or kill the fans' trigger, which is the real overheating risk.
How much does it cost to fix P0118?
The fix most associated with this code — coolant temperature sensor — runs $58 – $170 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0118?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0118 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0118?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0118?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.