Fix P0117: Mechanic’s solutions that WORKED

The code P0117: Engine Coolant Temperature Sensor Circuit Low Input indicates a fault in the coolant temperature sensor’s electrical circuit. This code specifically suggests that the signal it’s sending to the engine’s computer is unrealistically low, and most often, it is caused by a malfunctioning coolant temperature sensor or issues in its wiring circuit.

P0117 quick overview

Meaning	P0117: Engine Coolant Temperature Sensor Circuit Low Input
Is it serious?	Yes. If the ECU cannot accurately determine the engine’s temperature, it may cause poor performance, increased emissions, poor fuel economy, or engine damage.
Possible causes	– Faulty or failing engine coolant temperature sensor. – Contaminated coolant. – Wiring or connection issues (shorts, corrosion, or disconnections). – Faulty engine control module (less common).
How to diagnose?	1. Use a scanner to confirm the P0117 code and check for any other related codes. 2. Visually inspect the engine coolant sensor and its wiring for visible damages or disconnections. 3. Measure the resistance of the coolant temperature sensor. Compare readings to the manufacturer’s specifications. 4. Check the coolant’s condition and level. Replace if contaminated or low. 5. Inspect the wiring and connections for shorts, corrosion, or disconnections. 6. If all else seems fine, consider testing or replacing the engine control module.

How to fix p0117 – learn from mechanics

I looked into P0117 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Ford

Vehicle Information:
2000 Ford Explorer XLT 4.0L

Issue + Repair History:

• Customer complaints: High idle, engine overheating, and check engine light on.
• Diagnostic code: P0117 (ECT circuit low).
• Engine seemed fine in the shop; no high idle was observed.
• After 5 minutes of running: Scanner ECT temperature read 258°F, but a temperature gun on the ECT read 195°F.
• Previous repair history: Replacement of plugs and wires at another shop.

Suggestions from the Community:

• Mechanic 1: Advised checking the engine for a blown head gasket.
• Mechanic 2: Suggested replacing the sensor and rechecking since the diagnostic tool only shows what the sensor reports.
• Mechanic 3: Mentioned that the engine might heat up quickly due to a possible cylinder head gasket leak and indicated the diagnostic code could mean there’s a bad sensor.
• Mechanic 4: Recommended testing the radiator for head gasket issues and questioned whether the engine truly heats up to 258°F in 5 minutes.

Final Fix:
The problem was traced to a bad thermostat that caused the engine to overheat and a malfunctioning ECT sensor that provided incorrect temperature readings. The sensor had a loose part. Tests were conducted to check the head gasket, but no problems were found. After replacing the thermostat and ECT sensor, the issue was resolved.

Case #2: Smart

Vehicle Information:
2009 Smart Fortwo Brabus 1.0L

Issue + Repair History:

• Problem: Cooling fan works intermittently, and codes P0116 and P0117 are stored.
• The customer mentioned needing wiring diagrams and the cooling fan relay location.
• Reference to misinformation from a source called Alldata.

Suggestions from the Community:

• Mechanic 1: Explained that there’s no fan relay, as it’s integrated into the fan itself. The front SAM (Signal Acquisition Module) activates the fan using a pulse signal, and the fan operates based on demand. The fault codes typically indicate a faulty fan with its integrated control unit.

Final Fix:
After further inspection, two wires linked to the radiator fan were identified: Brown (ground) and Red with a Blue stripe (12-volt supply). The latter was traced through the firewall to the SAM. The SAM was removed to check its connection point in an 8-pin connector. A diagnostic tool (Maxidas) was used to activate the cooling fan, and it was found that the fan pulsed, which was initially thought to be normal but was later corrected. Ultimately, the issue was traced to the cooling fan itself. The provided reference (Alldata) had incorrect relay positioning, and it was determined that the relay is internal to the SAM.

Case #3: BMW

Vehicle Information:
1999 BMW 740i 4.4L

Issue + Repair History:

• Problem: The MIL (Malfunction Indicator Lamp) turns on, especially at full throttle.
• The potentiometers were tested and appeared to be within specifications.
• Previous Repair History: The vehicle had a faulty cam position sensor. After replacement, full-throttle usage caused the vehicle to enter failsafe mode and produced specific codes.

Suggestions from the Community:

• Mechanic 1: Asked about the scan tool used to detect the codes, implying the mentioned codes typically relate to coolant temperature.
• Mechanic 2: Clarified that the BMW system for this model is ME 7.2 and that the provided codes P0117 and P0119 are generic SAE codes related to engine coolant temperature, not the throttle potentiometer.
• Mechanic 3: Suggested checking a specific site for classifieds, though this seemed unrelated to the primary issue.

Final Fix:
The technician cleared the adaptations, which reset the “fly by wire” system, an electronic system controlling the throttle. After doing this reset, the warning light issue was resolved, and the vehicle functioned normally.

Case #4: Volkswagen

Vehicle Information:
1999 Volkswagen Beetle GL
Engine: 2.0L

Issue + Repair History:

• Problem: Poor idle quality and MIL lamp on.
• Previous repair: Crankshaft sensor was replaced due to a no-start issue.

Suggestions from the Community:

• Address the issue of the coolant temperature sensor (CTS) reading -46°C. Consider using a dealer-sourced sensor.
• Clean the throttle body and re-adapt it using Vag-com.
• Check the wiring and resistance for the coolant temperature sensor circuit.
• Investigate the timing belt, as it might be out of time or need replacement.
• Check the grounds for the circuits, especially at the PCM.
• Ensure the thermostat and water pump are functioning correctly.

Final Fix:
The mechanic discovered that multiple problematic parts shared a common ground wire, which showed higher resistance under load. The solution involved adding an extra ground to this wire and soldering it in place. After this, the throttle did not need relearning, and the car’s issues were resolved.

Case #5: Audi

Vehicle Information:
2000 Audi TT
Engine: 1.8L

Issue + Repair History:

• Problem: Extended crank time before engine starts, and the engine chugs when throttle is applied when cold (only after sitting for about 6-8 hours).
• Previous Repair: The crank sensor was replaced, but the condition persisted.
• Diagnostic Notes: All fuel pressure readings were within specs. During a no-start condition, there was no spark. Code P0117 indicating an engine coolant temp sensor signal too low intermittently (G62) was found.
• Observations: Both coolant temp sensors showed coolant contamination.

Suggestions from the Community:

• Replace the coolant temp sensor (CTS) as they are prone to failure.
• Inspect if the primary ignition circuit or the ECM coil triggers are missing.
• Check the fuel volume.
• Investigate the no-spark condition during cranking, considering possible failures in the output stage or coils.
• If replacing the coolant temp sensors, ensure the use of updated sensors (the newer ones are green, while older versions could be black or blue).
• Compare the readings between the ECU coolant temp and the instrument cluster temp gauge. The CTS can cause starting problems if it sends incorrect signals to the ECU about fuel mixtures.

Final Fix:
Both coolant sensors (coolant temp and fan switch sensors) were replaced. After the replacement, the vehicle started without issues in cold conditions. The client reported no problems after driving the car for about two weeks.