P0117 Code: Engine Coolant Temperature Sensor 1 Circuit Low
The coolant temperature signal is pinned near zero volts, which the PCM reads as an impossibly hot engine.
At a glance
Severity: Low
- Meaning
- Engine Coolant Temperature Sensor 1 Circuit Low
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $0–$1,500
- DIY difficulty
- Easy
The coolant temperature sensor is a thermistor fed with a 5-volt reference: the hotter the coolant, the lower its resistance and the lower the voltage the PCM sees. P0117 sets when that voltage drops below the lowest value a real engine could produce, usually under about 0.1 to 0.2 volts, for more than a couple of seconds. The PCM then believes the engine is scorching hot, often showing 250°F or more in live data even on a cold morning. The signal wire is being pulled to ground somewhere: inside a failed sensor, through a chafed harness or through coolant that has crept into the connector and bridged the pins. Because the PCM trusts the reading, it cuts cold-start enrichment, runs the fans flat out and may shut the A/C compressor off to protect the engine.
Can I keep driving? You can drive, but cold starts may be rough or need several tries because the PCM thinks the engine is already warm. The fans running constantly and the A/C dropping out are annoying rather than dangerous. Get it fixed before winter.
How to read P0117
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 17 Specific fault
Symptoms
- Check engine light on
- Live data shows coolant temperature pegged at a maximum value such as 250°F right after a cold start
- Hard starting or stalling on cold mornings, then fine once warm
- Radiator fans roar at full speed from the moment the engine starts
- A/C stops blowing cold, or the compressor won't engage
- Temperature gauge pegged hot on vehicles that drive the gauge from the same sensor
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Sensor shorted internally, often after coolant got past the seal into the element or the housing cracked
- Signal wire chafed through and touching ground, typically where the harness runs near the exhaust manifold or a bracket
- Coolant or water inside the connector bridging the signal and ground terminals
- Wrong sensor installed (single-wire gauge sender in the PCM sensor position, or an incorrect part)
- Signal wire pinched during other repair work (timing cover, intake or head gasket job)
- PCM input fault or internal short
Diagnosis step by step
Confirm what the PCM sees
With the key on and the engine cold, read coolant temperature in live data. A value pegged at the top of the scale, far above anything the engine could reach, confirms the low-voltage fault rather than a real overheat. If the temperature reads believable but the code is stored, the short is intermittent, so plan a wiggle test.
Tools: OBD-II scanner
Unplug the sensor and watch the reading flip
Disconnect the sensor connector with the key on. If the live-data value immediately swings to the opposite extreme, around -40°F, the wiring and PCM are fine and the sensor itself is shorted. If the reading stays pegged hot with the sensor unplugged, the short is in the harness or the PCM.
Tools: OBD-II scanner
Measure the sensor's resistance
With the connector off, measure across the two sensor pins. A healthy sensor reads in the thousands of ohms cold and a few hundred ohms warm. A reading near zero ohms, or one that collapses when you tap the sensor, confirms an internal short. Also check from each pin to the sensor body: there should be no continuity.
Tools: Multimeter
Check the reference voltage at the connector
Still unplugged, key on, measure the signal terminal in the harness connector to battery negative. You should see close to 5 volts. Zero volts means the signal wire is grounded somewhere upstream, or a reference-voltage problem that will usually set codes on other sensors too.
Tools: Multimeter
Trace the harness for the short
Follow the two wires from the sensor back toward the main harness, paying attention to anywhere they pass over the exhaust manifold, under a bracket or through a clip. Look for melted insulation, rub-throughs and green corrosion in the connector. Wiggle sections while watching live data to catch an intermittent contact.
Tools: Flashlight, OBD-II scanner
Replace the sensor, then verify the fix
Fit the correct sensor with a new seal or thread sealant as specified, top off any coolant lost and clear the code. On the next cold start the live-data temperature should match the intake air temperature and then climb normally; the fans should stay off until the engine is up to temperature.
Tools: OBD-II scanner, Gloves
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace coolant temperature sensor | Yes | $60–$200 |
| Replace corroded connector pigtail | Yes | $30–$150 |
| Repair chafed wiring and re-route harness | No | $100–$350 |
| Clean connector and refresh seals | Yes | $0–$40 |
| Replace PCM (rare) | No | $600–$1,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
With the check engine light on, the car fails an OBD-II emissions inspection before the tailpipe is even considered. After the repair, clear the code and drive a few days so the readiness monitors, which need a valid warm-up, can complete.
Frequently asked questions
Why does my engine start badly when the code means it thinks it is hot?
A cold engine needs extra fuel to start, the same job a choke used to do. Because the PCM believes the coolant is already at 250°F, it withholds that enrichment. The mixture is too lean for a cold cylinder, so the engine cranks longer, stumbles or stalls until it really warms up.
Are the fans running all the time going to hurt anything?
Not the engine, but the fans draw a lot of current and put extra load on the alternator and battery. On some vehicles the PCM also disables the A/C compressor as a precaution, so the cabin stays warm in summer until you fix the sensor.
Is P0117 ever a real overheating problem?
Almost never. A real overheat produces a gradual rise you can watch in live data and sets P0217 instead. P0117 is an electrical reading below what any coolant temperature could produce. If the engine also smells hot or is losing coolant, treat that as a separate problem.
Could I have shorted it while doing other work?
Yes, and it is a common story. The sensor wires often run near the intake manifold, timing cover or alternator, and a pinched wire or a connector that got coolant in it during a thermostat or water pump job shows up as P0117 on the first drive afterward.