P0115
PowertrainEngine Coolant Temperature Sensor Circuit Malfunction
This is usually a small job and a cheap part, so don't panic. The coolant temperature sensor tells the ECU how hot the engine is, and the ECU leans on that reading to set fuelling, idle, fan operation and cold-start enrichment. P0115 logs when the voltage coming back from that sensor goes out of range, either too high or too low to make sense. The sensor itself fails far more often than the wiring, and they're typically £10 to £40 on most popular cars.
ⓘ Information only. This page provides general educational information about fault code P0115. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0115 mean?
P0115 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Engine Coolant Temperature Sensor Circuit Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, sometimes with a temperature gauge that reads stone cold or pinned high
- • Hard starting from cold, then it behaves itself once warm
- • Cooling fans running flat out the moment you turn the key, or never coming on at all
- • Black smoke from the exhaust and a noticeable thirst, because the ECU thinks the engine is cold and dumps in extra fuel
- • Rough running and hesitation during the warm-up period
- • Worse fuel economy that doesn't recover even on a long motorway run
Possible causes
Causes commonly associated with P0115, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed coolant temp sensor, the usual culprit. The internal thermistor goes open or short circuit and sends a daft voltage. These are cheap items that age out
- 2. Corroded or loose connector at the sensor. Coolant weeps past the plug, or the locking tab cracks and the pins back out, giving an intermittent reading
- 3. Damaged wiring in the loom to the sensor, chafed against a bracket or melted near the manifold heat
- 4. Poor earth on the sensor circuit, which skews the reading high
- 5. Low coolant or air in the system so the sensor tip isn't sitting in liquid and reads erratically
- 6. A sticking thermostat that holds the engine cold can throw a low-temp code, though that's more often P0116 or P0128 than P0115
- 7. Faulty ECU, but that's rare and the last thing to suspect after everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Plug in a scan tool and look at the live coolant temp value with the engine dead cold. If it reads -40C or +130C with a stone-cold engine, the sensor or its circuit is reporting nonsense and that's your direction
- 2. Unplug the connector and have a proper look. Green crusty pins, coolant inside the plug, or a wire pulled out of the back are all common and cost nothing to find
- 3. Back-probe the sensor connector with a multimeter, ignition on, and check for the 5V reference. No reference voltage points at wiring or ECU, not the sensor
- 4. Measure the sensor's resistance across the temperature range and compare to the figures in the manual. A cold sensor should read a few thousand ohms, dropping to a couple of hundred when hot. Way out, or reading open, means a new sensor
- 5. Check the coolant level and bleed the system if it's low or aerated, since a sensor tip sitting in air gives rubbish readings
- 6. Compare the coolant temp reading against the intake air temp sensor first thing on a cold morning. Both should agree closely before the engine fires
Common questions about P0115
How do I know if it's the sensor or the wiring behind it? +
Quickest test is the multimeter at the connector. Unplug the sensor and check resistance across its two pins with the engine cold, then again once warm. If the reading is open circuit, dead short, or nowhere near the manual's figures, the sensor is gone. If the sensor measures fine on its own, the fault is in the loom or connector, so check for the 5V reference and a clean earth at the plug. Wiggle the wiring while watching live data on a scan tool, an intermittent reading that jumps about as you move the loom tells you exactly where the break is.
Can I just swap the sensor myself? +
For most owners with basic spanners, yes. The sensor usually screws into the cylinder head, thermostat housing or a coolant rail, and you'll find it with a single connector clipped on. Catch the coolant you lose in a tray, fit the new sensor with a smear of fresh coolant on the thread, top up and bleed the system, then clear the code. The one job to be careful with is sensors mounted low down, where you'll lose more coolant and might want the system topped and bled properly. Don't fit the cheapest unbranded sensor going, a known brand for £15 to £30 saves you doing it twice.
If I clear the code, will it stay gone? +
Only if you've actually fixed the cause. Wipe a genuine sensor or wiring fault and the code will be back within a drive cycle or two, often before you've left the street. The exception is a one-off glitch from a connector that wasn't seated properly, reseat it, clear the code, and it may not return. If P0115 keeps coming straight back after clearing, stop clearing it and start measuring. Driving on a dud coolant reading for weeks makes the engine run rich, sooting up the plugs and slowly cooking the catalytic converter.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →