Faulty coolant temperature sensor with internal short or drifted resistance; Signal wire chafed and shorted to earth or to a sensor earth return; Water or corrosion in the sensor connector or terminals causing a low-resistance path; Wrong or non-suitable replacement sensor fitted previously; Fault inside the engine control unit's sensor input circuit; Wiring harness damage elsewhere along the loom, such as rubbing on engine mounts or brackets.

P0117

Engine Coolant Temperature Sensor 1 Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
17: FaultThe specific fault in that area

What it means

P0117 means the engine control unit has seen the signal voltage from the engine coolant temperature sensor circuit fall below the expected range, which it reads as an impossibly high coolant temperature. This usually points to a short to earth in the signal wiring, a faulty sensor or a connector problem. The unit typically substitutes a default temperature value while the fault is present.

Symptoms

Engine warning light on
Hard starting or long cranking when the engine is cold
Cooling fan running constantly or at the wrong times
Rough idle or poor running, especially when cold
Higher fuel consumption or black smoke from rich running
Temperature gauge reading wrongly or not moving

Possible causes

Faulty coolant temperature sensor with internal short or drifted resistancelikely
Signal wire chafed and shorted to earth or to a sensor earth returnlikely
Water or corrosion in the sensor connector or terminals causing a low-resistance pathpossible
Wrong or non-suitable replacement sensor fitted previouslypossible
Fault inside the engine control unit's sensor input circuitrare
Wiring harness damage elsewhere along the loom, such as rubbing on engine mounts or bracketspossible

How to diagnose

Read and note the freeze frame data and any other stored codes, especially those for other temperature sensors or reference voltage faults
With the engine cold, compare the live coolant temperature reading with the actual temperature, which should be close to ambient or intake air temperature
Inspect the sensor connector and nearby wiring for water, corrosion, bent pins, melted insulation or chafing
Unplug the sensor with the ignition on and watch the live data; the reading should swing to a very low temperature value, and if it stays high, suspect a wiring short to earth
Measure the sensor resistance at a known temperature and compare it with the manufacturer's specification, since values vary by model
Check the signal circuit for a short to earth and continuity back to the control unit with the connectors disconnected
Check the reference voltage and earth at the sensor connector against the wiring diagram for that vehicle
Only if wiring and sensor are proven good, test the control unit input following the manufacturer's procedure

How to fix

Replace the coolant temperature sensor with the correct specification part
Repair or replace the damaged signal wire or section of loom, then protect it from rubbing
Clean, dry and repair the connector terminals, or replace the connector if corroded
Correct any incorrect sensor type previously fitted
Repair or replace the engine control unit only after all other causes are ruled out

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but poor cold running, a constantly running fan and extra fuel use are likely. Because the unit cannot read true coolant temperature, have it checked soon and watch the real temperature gauge or any overheating signs.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.