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What code does a bad engine coolant temp sensor throw?

A bad engine coolant temperature (ECT) sensor can trigger several diagnostic trouble codes (DTCs) in a vehicle's onboard computer system. The specific code thrown will depend on the nature of the problem with the ECT sensor, but common codes include P0115, P0116, P0117, and P0118.

Diagnostic Trouble Codes for a Bad ECT Sensor

Here are some of the most common diagnostic trouble codes associated with a faulty engine coolant temperature sensor:

P0115 - Engine Coolant Temperature Circuit Malfunction - This code indicates a problem with the ECT sensor circuit, such as a short or open circuit.

P0116 - Engine Coolant Temperature Range/Performance Problem - This code suggests the ECT sensor is not providing accurate temperature readings, possibly due to a sensor failure or improper calibration.

P0117 - Engine Coolant Temperature Sensor Circuit Low Input - This code indicates the ECT sensor is reading a temperature that is lower than expected, which could mean the sensor is faulty or the wiring is damaged.

P0118 - Engine Coolant Temperature Sensor Circuit High Input - This code suggests the ECT sensor is reading a temperature that is higher than expected, which may be caused by a sensor failure or wiring issue.

These are some of the most common diagnostic trouble codes related to a bad engine coolant temperature sensor. The specific code thrown will depend on the nature of the problem with the sensor.

Will a coolant temperature sensor throw a code?

Yes, a faulty coolant temperature sensor can absolutely throw a check engine light and store Diagnostic Trouble Codes (DTCs). When the sensor provides inaccurate readings to the car's computer, the computer will trigger a code to indicate a problem with the sensor or its circuit.
This video explains the check engine light codes related to a faulty coolant temperature sensor: 59sTRQYouTube · Aug 18, 2023
Common trouble codes

P0115: Engine coolant temperature sensor circuit malfunction
P0116: Engine coolant temperature sensor circuit range/performance
P0117: Engine coolant temperature sensor circuit low input
P0118: Engine coolant temperature sensor circuit high input
P0119: Engine coolant temperature sensor circuit intermittent
P0125: Insufficient coolant temperature for closed loop operation

What happens when the sensor fails

Incorrect fuel mixture: The computer might think the engine is colder than it is and inject too much fuel, causing a "rich" condition.
Poor fuel economy: The engine running too rich can waste fuel and lead to poor gas mileage.
Hard or rough starts: The incorrect air-fuel ratio can make the engine difficult to start or cause it to run rough.
Engine overheating: If the sensor fails to report the correct high temperature, the cooling fan may not turn on when needed.
Stalling: The engine could stall if the computer sprays too much fuel, potentially fouling the spark plugs.

You can watch this video to learn about the signs and symptoms of a faulty engine coolant temperature sensor: 52s8020 AutomotiveYouTube · Mar 2, 2025

Can I still drive my car with a P0128 code?

You can drive a short distance with code P0128, but you should get the issue fixed as soon as possible. While it's not a critical emergency like overheating, a P0128 code means the engine isn't reaching its optimal operating temperature, which can lead to reduced fuel efficiency, increased emissions, and potential long-term engine wear.
This video explains what happens when you drive with a P0128 code: 59sMake It Easy MechanicYouTube · Jun 10, 2023
Risks of driving with a P0128 code

Reduced fuel economy: The engine computer may run a richer fuel mixture for a longer warm-up period, which wastes fuel.
Lower performance: The engine is not operating at its peak efficiency, which can negatively affect performance.
Increased wear: A colder engine is less efficient with its lubrication, which can lead to increased wear on engine components over time.
Emissions issues: Running too cold can increase emissions, and in some cases, lead to a failed emissions test.
Potential for more damage: Although less common, prolonged driving with a cold engine can lead to other problems, such as a fouled spark plug or damage to the catalytic converter.
Poor cabin heat: The heater and defroster may not work as effectively.

What to do

Don't ignore it: Get the issue diagnosed and repaired promptly.
Consider professional help: A mechanic can quickly diagnose the root cause, which is often a faulty thermostat, and replace it.
Drive conservatively: Avoid long-distance trips if possible until the car is fixed.

Is engine code P2181 serious?

High or low engine temperature or a faulty thermometer are the usual causes of trouble code P2181. Overheating can ruin an engine in a matter of minutes, whereas an engine running too cool could be slightly less urgent. Try to normalize engine temperature as soon as possible.

How do I know if my engine coolant temperature sensor is bad?

Symptoms of a bad coolant temperature sensor include an erratic temperature gauge, engine overheating, illuminated check engine light, poor fuel economy, and a rough idle or stalling. Other signs can be the cooling fan not working correctly, black smoke from the exhaust, and difficulty starting the engine.
Performance and operational symptoms

Engine overheating: The most critical symptom, as the sensor failing can prevent the engine from properly engaging the cooling fan.
Erratic temperature gauge: The needle may show a constantly high, low, or fluctuating temperature.
Rough idle and stalling: The engine's computer uses the sensor's data to calculate the right air-fuel mixture. An incorrect reading can cause the engine to run poorly, idle roughly, or stall, especially when cold.
Poor fuel economy: The car may use more fuel than it should because the engine computer is getting incorrect information about engine temperature and is adjusting the air-fuel mixture incorrectly.
Difficulty starting: The engine may be hard to start, especially when warm, due to the wrong amount of fuel being injected.

Exhaust and emissions symptoms

Black smoke from exhaust: This indicates the engine is running too rich (too much fuel), which can be caused by the sensor sending a faulty "cold" signal.
Increased emissions: The incorrect air-fuel mixture can lead to higher emissions, potentially causing your car to fail an emissions test.

Other warning signs

Check engine light: The engine control unit (ECU) will trigger the light if it detects a problem with the sensor's data.
Cooling fan malfunction: The radiator fan may not turn on when the engine gets hot, or it may run constantly.
Poor engine performance: You may experience reduced power, misfires, or engine pinging.

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.