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Home > Fault Codes > P0116

P0116 — Engine Coolant Temperature Sensor Range/Performance

By Mr Auto Fixer — Professional Mechanic, 20+ Years Experience

The engine coolant temperature sensor reading is technically valid, but the temperature pattern doesn't match what the ECU expects during warm-up.

Low — Fix When Convenient
Last checked: May 2026

What Is P0116?

P0116 is a rationality fault on the Engine Coolant Temperature (ECT) sensor — the reading is within its electrical range but does not match what the ECU expects based on other inputs such as intake air temperature, elapsed run time, and warm-up progression. Unlike P0117 or P0118 which indicate an electrical fault, P0116 means the sensor is producing a reading that does not make sense in context.

Common examples include a coolant temperature that does not rise after extended running (suggesting the sensor is stuck at ambient) or a temperature that rises and falls erratically. The ECT sensor is critical for fuel enrichment during warm-up, idle speed control, cooling fan activation, and ignition timing. A plausible but inaccurate sensor causes subtle fuelling problems that are hard to diagnose without live data.

Common Symptoms

  • Engine management light on
  • Possible poor fuel economy (extended cold enrichment)
  • Cooling fan may activate at wrong times
  • Slight rough running during warm-up
  • In some cases no obvious driveability symptoms
  • Possible hard cold starting

Common Causes

Failing ECT SensorThe sensor produces an intermittent or inaccurate reading within its valid range. The resistance element degrades but does not fail completely, so it appears plausible to the ECU circuit check but fails rationality tests.
Air Pocket in Cooling SystemAn air pocket trapped near the ECT sensor gives the sensor a false reading — it measures air temperature rather than coolant temperature. The reading changes erratically as coolant flow shifts the air pocket.
Low Coolant LevelA low coolant level can expose the ECT sensor tip to air or steam intermittently, causing irrational temperature readings.
Thermostat FaultA stuck-open thermostat prevents the engine reaching normal operating temperature. The sensor is working correctly but reading an abnormally cool engine — which the ECU flags as implausible for the run time elapsed.
Wiring Intermittent FaultA partially corroded connector or intermittent wiring fault causes the sensor reading to jump around within range rather than failing outright.

How to Diagnose P0116

1

Check Coolant Level

Inspect the coolant reservoir and radiator. Low coolant can expose the ECT sensor, causing erratic readings. Top up if low and bleed any air from the system by running the engine with the heater on full.

2

Monitor ECT in Live Data

Watch coolant temperature on the OBD scanner from cold start. It should rise steadily, reaching 80–95°C within 5–10 minutes. A temperature that stays at ambient after 10 minutes indicates a stuck-open thermostat or stuck sensor. An erratically jumping temperature points to an air pocket or wiring fault.

3

Check the Thermostat

If the temperature never reaches 80°C, the thermostat is likely stuck open. Confirm by feeling the top radiator hose — it should only become hot once the thermostat opens (at 80–88°C). If the top hose is hot from startup, the thermostat is open permanently.

4

Bleed the Cooling System

Burp the cooling system of air pockets by running the engine with the heater on maximum and the reservoir cap off (with the engine cold). Watch for bubbles in the reservoir and top up as needed until the level stabilises.

5

Replace ECT Sensor

If coolant level, thermostat, and air bleeding are all normal but the reading remains erratic, the sensor has degraded. ECT sensors are typically £10–£30 and straightforward to replace.

Verdict

Start by checking coolant level and thermostat operation — both are free to check and are common causes of P0116. If cooling system condition is normal, replace the inexpensive ECT sensor. Do not overlook the thermostat — a stuck-open thermostat causes exactly the pattern that triggers P0116.

Mr Auto Fixer
Written by
Mr Auto Fixer
Qualified Mechanic20+ Years ExperienceUK Based

Professional UK mechanic with over 20 years of hands-on experience. All guides are based on real workshop repairs — not theory.

About Mr Auto Fixer
Common Questions

Frequently Asked Questions

Yes. If the ECT sensor reads falsely cold, the ECU applies excess cold-start enrichment for too long, wasting fuel. A stuck-at-ambient sensor means the engine never enters lean closed-loop operation.
A stuck-open thermostat allows coolant to circulate through the radiator continuously, preventing the engine reaching normal temperature. The ECU expects the temperature to reach 80°C within a set time after startup — if it does not, it logs P0116.
No, though a blown head gasket can cause coolant contamination and air pockets that trigger P0116 as a secondary symptom. Check for white exhaust smoke, mayo-coloured oil filler cap residue, or coolant loss without obvious external leaks.
Yes. The ECT is used for fuel enrichment, idle speed, ignition timing, and cooling fan control. An irrational ECT reading degrades all of these, though the effects may be subtle.
It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.
Related

Related Fault Codes

P0117
ECT Sensor Low Input — sensor reading too cold
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P0118
ECT Sensor High Input — sensor reading too hot
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P0128
Coolant Temperature Below Thermostat Regulating Temperature
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