OBD-II trouble code
P0118: Engine Coolant Temperature Sensor Circuit High Input
The ECT sensor signal is reading higher than the PCM expects — usually an open circuit, a broken sensor, or a disconnected connector that makes the engine appear permanently cold.
Quick facts
- System
- Powertrain
- Category
- Sensors / Cooling
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $400
- DIY difficulty
- Beginner DIY
What does P0118 mean?
P0118 sets when the powertrain control module sees a voltage on the coolant temperature signal wire that is higher than the calibrated maximum. Because the sensor is a thermistor whose resistance climbs as the coolant cools, a wide-open circuit looks identical to a sensor sitting in liquid nitrogen — the PCM interprets the missing signal as the engine being unrealistically cold (often -40°F).
The practical fallout is that the PCM never declares the engine warm. Cold-start enrichment stays active well past the point it should drop out, closed-loop fuel control may never engage, and the EVAP system, EGR, and cooling fan logic all behave as if the car is still warming up. Drivers usually notice a rich-smelling exhaust, sooty tailpipe deposits, fuel economy that has fallen off a cliff, and sometimes a faint stumble at idle once the engine is actually hot.
Left unrepaired, P0118 will eventually start damaging the catalytic converter. Continuous over-fueling sends unburned fuel into the cat, which raises its operating temperature and shortens its life. That's why this code shouldn't be ignored even though the car drives normally — fixing the sensor cheaply now is much better than chasing a P0420 later.
Common causes
- ECT sensor connector unplugged or pushed off during recent service
- Internally failed sensor (open thermistor element)
- Broken signal wire between the sensor and the PCM
- Corroded terminals in the sensor connector
- Damaged connector body, often from a heat shield being removed and reinstalled
- Ground circuit fault on the sensor's return wire
- Rodent damage to the engine harness near the sensor
Symptoms
- Check engine light on with P0118 stored
- Strong fuel smell from the exhaust, especially at idle
- Visible black exhaust soot at the tailpipe
- Noticeable drop in fuel economy
- Temperature gauge reading low or sitting at the bottom of the scale
- Cooling fans never coming on at idle, even after a long drive
- Eventual P0420 or fuel-trim codes if the issue is ignored for months
Diagnostic steps
- 1.Confirm the sensor reading at key-on. A value pinned at -40°F is the textbook open-circuit signature for this code.
- 2.Visually inspect the connector — a back-popped pin or unplugged connector is one of the most common P0118 causes after recent engine work.
- 3.With the connector unplugged, jumper the two pins together. The reading should jump to about 300°F. If it does, the sensor is the failed component.
- 4.If the reading stays pinned cold with the jumper in place, you have an open in the signal or ground wire between the connector and the PCM.
- 5.Check connector terminals for corrosion or spread pin tension — these issues come and go and can cause intermittent P0118 codes.
Repair cost
$100 – $400
Sensor replacement on an accessible engine typically lands around $100-$180. Cost rises when the sensor is buried under the intake manifold or when a connector pigtail and harness repair are needed. Catalytic converter damage from extended driving with P0118 active is not included in this range — that's a separate $1,000+ problem if it develops.
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Open the Repair Cost Estimator with check engine light diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is a beginner-friendly repair. Common hand tools, a free afternoon, and a willingness to follow a procedure are usually enough. The risk of causing a bigger problem is low if you read up on your specific vehicle first.