Failed coolant temperature sensor (OBD-Codes.com says this is the usual cause). Bad or corroded connection at the sensor. Open in the ground circuit between the sensor and the PCM. Short in the voltage feed between the sensor and the PCM. Rusty or corroded engine coolant (listed by RepairPal). Faulty PCM (less likely).

Clean or repair the sensor connector. Repair an open or shorted ECT wire between the sensor and the PCM. Replace the coolant temperature sensor. Flush and refill rusty or corroded coolant. PCM repair or replacement only after the sensor and wiring are confirmed good.

P0118🔧 Engine cooling / coolant temperature sensorStop and fix promptly

P0118: Engine Coolant Temperature (ECT) Sensor 1 Circuit High (Meaning, Causes, Fixes)

Q: What does P0118 mean?

P0118 means the engine computer is seeing a coolant temperature sensor signal voltage that is too high, which the sensor reports as very cold coolant. The usual causes are a failed coolant temperature sensor, a bad connection, an open or shorted wire in its circuit, or, per RepairPal, rusty or corroded coolant. The engine coolant temperature (ECT) sensor is a thermistor screwed into a coolant passage, usually in the cylinder head. OBD-Codes.com explains that the PCM (engine computer) supplies a 5-volt reference and a ground, and the sensor's resistance is high when the coolant is cold and drops as it warms up. RepairPal adds that the signal voltage is high when the engine is cold and low once it warms up, and that the PCM uses the reading for the air-fuel ratio, ignition timing, the cooling fans and many emission controls. P0118 sets when that signal voltage is higher than expected. OBD-Codes.com says this happens if the sensor reads below freezing after the engine has run for more than a few minutes, or if the sensor resistance is out of spec. Related ECT codes include P0115, P0116, P0117 (circuit low) and P0119 (intermittent).

Q: Can I keep driving with P0118?

Stop and fix promptly. Get this diagnosed promptly. RepairPal rates P0118 'Urgent', meaning it can lead to severe damage or be unsafe, partly because the engine computer also uses this sensor to protect the engine from overheating and to run the cooling fans. Sources differ on how it feels: RepairPal says many cars show no symptoms beyond the light, while OBD-Codes.com lists a possible no-start, rough running, misfires and black smoke. Watch the temperature gauge, and stop driving if it reads high, the engine overheats, or it runs very rough.

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0118 means

The engine coolant temperature (ECT) sensor is a thermistor screwed into a coolant passage, usually in the cylinder head. OBD-Codes.com explains that the PCM (engine computer) supplies a 5-volt reference and a ground, and the sensor's resistance is high when the coolant is cold and drops as it warms up. RepairPal adds that the signal voltage is high when the engine is cold and low once it warms up, and that the PCM uses the reading for the air-fuel ratio, ignition timing, the cooling fans and many emission controls. P0118 sets when that signal voltage is higher than expected. OBD-Codes.com says this happens if the sensor reads below freezing after the engine has run for more than a few minutes, or if the sensor resistance is out of spec. Related ECT codes include P0115, P0116, P0117 (circuit low) and P0119 (intermittent).

Severity: can I keep driving?

Stop and fix promptly

Get this diagnosed promptly. RepairPal rates P0118 'Urgent', meaning it can lead to severe damage or be unsafe, partly because the engine computer also uses this sensor to protect the engine from overheating and to run the cooling fans. Sources differ on how it feels: RepairPal says many cars show no symptoms beyond the light, while OBD-Codes.com lists a possible no-start, rough running, misfires and black smoke. Watch the temperature gauge, and stop driving if it reads high, the engine overheats, or it runs very rough.

Common symptoms

Check engine light on
Often no other symptoms (per RepairPal)
Hard starting or a no-start condition
Very poor fuel economy
Rough running, misfiring or black smoke in some cases (listed by OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Failed coolant temperature sensor (OBD-Codes.com says this is the usual cause)
Bad or corroded connection at the sensor
Open in the ground circuit between the sensor and the PCM
Short in the voltage feed between the sensor and the PCM
Rusty or corroded engine coolant (listed by RepairPal)
Faulty PCM (less likely)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes. Several sensor voltage codes at once (for example intake air temperature or MAP codes) can point to a shared wiring, ground or reference-voltage problem.
With the engine cool, check the coolant level and condition. RepairPal lists rusty or corroded coolant as a cause.
Find the coolant temperature sensor and check that its connector is fully seated, clean and not corroded, and that the wiring is not chafed or broken.
With a scan tool showing live data, check the coolant temperature reading. OBD-Codes.com says a sensible reading suggests an intermittent fault, so wiggle the connector and harness and watch for drop-outs.
If the reading is illogical (for example far below freezing on a warm engine), the sensor resistance and circuit need testing. OBD-Codes.com describes jumpering the connector terminals with a fused jumper wire to tell a sensor fault from a wiring fault.

Typical fixes

Clean or repair the sensor connector
Repair an open or shorted ECT wire between the sensor and the PCM
Replace the coolant temperature sensor
Flush and refill rusty or corroded coolant
PCM repair or replacement only after the sensor and wiring are confirmed good

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The temperature gauge reads oddly, the engine overheats, or the cooling fans do not behave normally
The code comes back after checking the connector and wiring
Testing needs a scan tool and a digital multimeter (checking the 5-volt reference, ground and continuity back to the PCM). OBD-Codes.com warns not to attempt PCM diagnosis if you are unsure how.
RepairPal warns the sensor is often replaced when the real cause is rusty coolant, a poor connection or chafed wiring, or a problem elsewhere in the cooling system

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0118?

Only carefully and not for long. RepairPal rates it 'Urgent'. Even if the car seems fine, the engine computer may not be getting a true coolant temperature, which it uses for fueling and fan control, so watch the gauge and get it diagnosed soon.

Does P0118 mean the coolant temperature sensor is bad?

Often, but not always. OBD-Codes.com says P0118 usually indicates a bad sensor, but both sources list wiring and connector faults, and RepairPal also lists rusty coolant and other cooling-system problems as common reasons a good sensor gets replaced.

What's the difference between P0117 and P0118?

P0117 is coolant temperature circuit low (signal voltage too low, usually reported as very hot coolant), and P0118 is circuit high (signal voltage too high, usually reported as very cold coolant).

Why do the sources describe the symptoms so differently?

RepairPal says many cars show no symptoms besides the check engine light, while OBD-Codes.com lists a possible no-start, rough running and black smoke. A falsely cold reading can make the engine run rich, so how bad it feels depends on the vehicle.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.