P0117

Powertrain
Generic (SAE)
Emissions

Severity

Diagnose soon

The engine runs, but the PCM now believes it is fully warm at all times. It skips cold-start enrichment, which causes hard cold starting, stalling and stumbling until the engine is genuinely warm. Cooling fans may run continuously, and on some vehicles A/C operation is inhibited. It moves higher if the temperature gauge is fed from the same sensor, because you lose your warning of a real overheat.

Safe to drive: Driveable in most cases, but expect hard cold starts. Watch the temperature gauge closely, since it may no longer be trustworthy.

Standard definition

Engine Coolant Temperature Sensor Low Input

In plain English

The signal wire from the coolant temperature sensor is reading too low a voltage, which the computer interprets as an impossibly hot engine. This is almost always an electrical problem in the sensor or its wiring, not an actual overheating engine — but confirm the engine is not overheating before assuming that.

The coolant temperature sensor is a negative temperature coefficient thermistor fed by a reference voltage through a pull-up resistor inside the PCM. As coolant warms, sensor resistance falls and the signal voltage drops with it. P0117 sets when that voltage falls below a threshold that corresponds to a temperature the coolant cannot physically reach. The usual causes are the signal wire shorted to ground, an internally shorted sensor, or coolant and corrosion inside a connector bridging the signal circuit to ground.

How the car detected it

The PCM samples the signal voltage on the coolant temperature circuit and compares it against a fixed low limit. Because the limit is calibrated above any temperature real coolant reaches before boiling out, any reading below it is treated as an electrical fault rather than a genuine measurement. Most strategies require the condition to be present for a continuous period before setting the code, to reject momentary noise.

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Can I keep driving?

Safe to drive: Driveable in most cases, but expect hard cold starts. Watch the temperature gauge closely, since it may no longer be trustworthy.
Urgency: Diagnose soon
Will it pass emissions: No
If ignored: Cold starts stay difficult and the engine stumbles until warm. Constantly running fans delay warm-up and add load the charging system has to carry; on vehicles whose fans keep running after key-off, they can flatten the battery overnight. Most importantly, if this sensor also drives your temperature gauge, you lose the warning that would tell you about a real overheat — and a missed overheat is head gasket and cylinder head territory.
Check engine light: Solid
Clearing the light: A completed drive cycle is normally required before the monitor re-runs and the light can clear.

Symptoms you may notice

Cooling fan runs constantly or not at all
Cabin heater weak or excessively hot
Possible long warm-up time or hard cold start

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

Likely causes

These are the causes this code can have. For the order to work through them in, see what to check first.

#	Cause	How often	Component
1	Signal wire shorted to ground	Very common	—
2	Sensor failed shorted internally	Common	—

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

What to check first

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

Whether the engine is actually overheating — coolant level, gauge behavior, steam, hot smell
The threshold for this code is normally set above any real coolant temperature, so it is almost always electrical. But confirming the engine is not genuinely cooking costs nothing and the downside of getting it wrong is severe.
The coolant temperature value the scan tool reports on a stone-cold engine
If a cold engine reports an extreme high temperature, the electrical fault is confirmed in seconds without a single tool touching the vehicle.
The sensor connector for coolant intrusion, corrosion and green residue
Coolant inside a connector conducts, bridging signal to ground and producing this exact fault. It is common and cheap to fix, and easily mistaken for a bad sensor.
The signal wire where it passes brackets, the engine block or exhaust heat shields
A chafed signal wire touching ground is a leading cause. Following the harness visually is faster than any electrical test.
Any recent work in the area, including coolant service or engine repairs
Pinched wiring and mis-seated connectors from recent work are a common source, and the timing is a strong clue.

How a shop diagnoses this

Read the reported coolant temperature on a cold engine
Tools Scan tool
With the engine cold and the key on, read the coolant temperature value. If it reports an extreme high figure while the engine is stone cold, the circuit fault is confirmed and you can move directly to isolating it.
Expected
A cold engine should report close to ambient. An extreme high reading confirms a low-voltage circuit fault
Unplug the sensor and watch the reading
Tools Scan tool
With the key on, disconnect the coolant temperature sensor connector and watch the scan tool value. Opening the circuit should let the signal rise to reference voltage, driving the reported temperature to its lowest value and typically triggering P0118 instead.
Expected
The reading swings to minimum temperature, often the bottom of the table. If it does, the sensor is internally shorted. If it stays hot, the fault is in the harness or the PCM
Measure the signal voltage at the harness connector
Tools Digital multimeter, wiring diagram
With the sensor unplugged and the key on, measure the signal terminal on the harness side against a known good ground. This should read near the module's reference voltage with the circuit open.
Expected
Near reference voltage, commonly around five volts on most systems. A reading near zero means the wire is shorted to ground
Test the signal wire for a short to ground
Tools Digital multimeter, wiring diagram
With the key off and both the sensor and the PCM connector disconnected, measure resistance from the signal wire to chassis ground. This isolates the wire from both ends so the result is unambiguous.
Expected
Open circuit. A low resistance reading confirms a short to ground in the harness
Inspect and test the sensor itself
Tools Digital multimeter, service information
Measure sensor resistance cold. An internally shorted sensor reads far below the value the chart calls for at that temperature, sometimes near zero. Warm the sensor and confirm resistance changes smoothly. The resistance chart is vehicle-specific.
Expected
Resistance matching the manufacturer's chart for the measured temperature
Check for coolant or corrosion inside the connector
Tools Flashlight, magnifier, contact cleaner
Pull both halves of the connector apart and inspect the terminals closely. Coolant residue, green corrosion or a milky film all indicate intrusion. Coolant in a connector conducts and creates a soft short that comes and goes with temperature.
Expected
Clean, bright, dry terminals with tight fit
Wiggle-test the harness under live monitoring
Tools Scan tool, assistant
With the scan tool watching the temperature value, flex the harness along its route, particularly at brackets, the block and any heat shield. An intermittent short shows up as a momentary jump to maximum temperature.
Expected
A rock-steady reading. Any jump localizes the damaged section
Verify the repair through a cold start and full warm-up
Tools Scan tool
Clear the code and let the engine cool completely. Then watch the temperature value climb smoothly from ambient to operating temperature. Confirm cold-start behavior is normal and the fans cycle correctly.
Expected
A smooth climb from ambient to operating temperature with no dropouts, and no return of the code

Possible repairs and what they cost

Repair	Only after you confirm	Typical cost	Who
Repair the signal wire shorted to ground	Low resistance from the signal wire to ground with both the sensor and the PCM disconnected	$120–$450	DIY
Clean or replace a connector contaminated with coolant or corrosion	Visible coolant residue or corrosion inside the connector, with the circuit reading correctly once cleaned and dried	$100–$320	DIY
Replace the engine coolant temperature sensor	The reading swinging to minimum when the sensor is unplugged, plus sensor resistance far below the chart value for its actual temperature	$90–$280	DIY
Repair a pinched or damaged harness section	An intermittent jump to maximum temperature during a wiggle test that localizes to a specific length of harness	$140–$500	DIY
Replace or repair the engine control module	A signal wire proven open to ground, a good sensor, and reference voltage still absent or pulled low at the module connector	$700–$2,000	Shop

When the sensor is the fault and it is accessible, this is one of the cheaper repairs on the car. Cost climbs when the sensor sits beneath an intake manifold, or when the fault is an intermittent short buried in a harness — tracing that is billed by the hour and can exceed the price of every part involved.

Common mistakes with this code

Replacing the sensor before unplugging it to see whether the reading changes. That one test separates a shorted sensor from a shorted wire in under a minute.
Assuming the engine is overheating and doing an unnecessary cooling system teardown. The code's threshold sits above real coolant temperature, so it points at the circuit — verify, then move on.
Missing coolant intrusion in the connector. A new sensor plugged into a contaminated connector fails immediately in the same way.
Repairing the chafed wire but leaving the routing unchanged, so it rubs through again at the same spot.
Confusing this with P0116. P0116 means the voltage is valid but the value is implausible. P0117 means the voltage itself is out of range — a different test path entirely.
Replacing the gauge sender instead of the PCM sensor on engines that have both. Which one is which varies by engine.

Components involved

Coolant Temperature Sensor
Coolant Temperature Sensor Wiring

Sources

Definition: Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
Diagnostic guidance: Written and reviewed in-house. See our editorial policy.
Review status: Reviewed

Found something wrong? Tell us — we publish corrections.

Whether the engine is actually overheating — coolant level, gauge behavior, steam, hot smell

The coolant temperature value the scan tool reports on a stone-cold engine

The sensor connector for coolant intrusion, corrosion and green residue

The signal wire where it passes brackets, the engine block or exhaust heat shields

Any recent work in the area, including coolant service or engine repairs

Read the reported coolant temperature on a cold engine

Unplug the sensor and watch the reading

Measure the signal voltage at the harness connector

Test the signal wire for a short to ground

Inspect and test the sensor itself

Check for coolant or corrosion inside the connector

Wiggle-test the harness under live monitoring

Verify the repair through a cold start and full warm-up