P0117🔧 Engine cooling / coolant temperature sensorStop and fix promptly

P0117: Engine Coolant Temperature (ECT) Sensor 1 Circuit Low (Meaning, Causes, Fixes)

Reviewed 2026-10-23 · Based on 2 cited source(s)

What P0117 means

The engine coolant temperature (ECT) sensor is a thermistor (a temperature-sensitive resistor) in a coolant passage of the engine. OBD-Codes.com explains that it is usually a two-wire sensor, with a 5-volt reference and a ground both supplied by the PCM (engine computer), and that its resistance is high when the coolant is cold and low when it is warm. RepairPal adds that the signal voltage is high when the engine is cold and falls as it warms up, and that the PCM uses the reading for the air-fuel ratio, ignition timing, the cooling fans and many emission controls. P0117 sets when the PCM sees a signal voltage below the sensor's normal operating range, which looks like an excessively hot engine. Related ECT codes include P0115, P0116, P0118 (circuit high) and P0119 (intermittent).

Severity: can I keep driving?

Stop and fix promptly

Get this diagnosed promptly. RepairPal rates P0117 'Urgent', meaning it can lead to severe damage or be unsafe, partly because the engine computer uses this sensor to protect the engine from overheating. A low signal reads as very hot coolant, and OBD-Codes.com points out that a real overheat can cause this code, so check that first. Sources differ on how it feels: RepairPal says many cars show no symptoms beyond the light, while OBD-Codes.com lists rough running, black smoke, no idle, or starting and then dying. Watch the temperature gauge, and stop driving if it reads high, the engine overheats, or it stalls or runs very rough.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Failed coolant temperature sensor (for example internally shorted)
  2. ECT signal wire shorted to ground, or the two sensor wires shorted to each other
  3. Faulty, damaged or corroded connectors, or loose terminals at the sensor or PCM
  4. Damaged wiring harness (chafed or melted wires)
  5. Rusty or corroded engine coolant (listed by RepairPal)
  6. Possibly a genuinely overheated engine (listed by OBD-Codes.com)
  7. Faulty PCM (less likely)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. First rule out a real overheat. OBD-Codes.com says that if the engine was overheating, diagnose that before anything else. With the engine cool, check the coolant level and condition; RepairPal lists rusty or corroded coolant as a cause.
  2. Read all stored codes. OBD-Codes.com notes that on many models the 5-volt reference is shared, so one shorted sensor can pull it low and set several sensor codes at once.
  3. Check that the coolant temperature sensor connector is fully seated, clean and not corroded, and look for chafed or melted wiring.
  4. With a scan tool, key on and engine off on a cold engine, compare the coolant temperature reading to the intake air temperature reading. OBD-Codes.com says they should match; RepairPal also says the two should move together.
  5. If the coolant reading is extremely hot (OBD-Codes.com gives more than 260 degrees F as an example), unplug the sensor. If the reading drops to the extreme cold value, OBD-Codes.com says the sensor is internally shorted; if it does not change, look for a short in the wiring.
  6. If the reading looks normal, the fault may be intermittent. Wiggle the harness and connectors while watching live data, and check freeze frame data for the reading when the code set.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The temperature gauge reads high, the engine is overheating, or the cooling fans do not behave normally
  • The code comes back after checking the connector and wiring
  • Testing needs a scan tool and a digital multimeter (checking the 5-volt reference, ground and the signal from the PCM). RepairPal suggests a test drive while watching live data and a careful heat-gun test of the sensor.
  • RepairPal warns the sensor is often replaced when the real cause is rusty coolant, a poor connection or chafed wiring, or a problem elsewhere in the cooling system

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0117?

Only carefully and not for long. RepairPal rates it 'Urgent'. Because the code reads as a very hot engine, first make sure the engine is not actually overheating, and stop if the gauge reads high or the engine stalls or runs badly.

Does P0117 mean the coolant temperature sensor is bad?

Often, but not always. Both sources list wiring and connector faults, RepairPal lists rusty coolant and other cooling-system problems as reasons a good sensor gets replaced, and OBD-Codes.com says a real overheat or another sensor shorting the shared 5-volt reference can also cause it.

What's the difference between P0117 and P0118?

P0117 is coolant temperature circuit low (signal voltage too low, reported as very hot coolant), and P0118 is circuit high (signal voltage too high, reported as very cold coolant).

Why do the sources describe the symptoms so differently?

RepairPal says many cars show no symptoms besides the check engine light, while OBD-Codes.com lists rough running, black smoke, no idle and stalling after starting. How bad it feels depends on how the engine computer reacts to a falsely hot reading on that vehicle.

Sources

  1. OBD-Codes.com: P0117 Engine Coolant Temperature (ECT) Circuit Low Input
  2. RepairPal: P0117 Engine Coolant Temperature Sensor 1 Circuit Low (rated Urgent)

Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.