P0117 means the engine computer is reading a voltage from the engine coolant temperature sensor that is too low to be real. Because of how the sensor works, a low voltage means the computer is being told the engine is impossibly hot, not cold. P0117 is almost never an overheating engine. It is an electrical fault, usually a short to ground in the sensor or its wiring, and two free checks with a scan tool can usually show which. This article covers what sets P0117, the six real causes in order of likelihood, how to tell a wiring short from a failed sensor, and what each repair typically costs.
Start with the table, then read the triage note under it.
| Cause | Telltale Sign | Typical Cost |
|---|---|---|
| Sensor signal wire shorted to ground | Reading stays pinned hot with the sensor unplugged; harness damage near the engine or exhaust | $50 to $300 |
| Failed coolant temperature sensor | Reading pinned hot, but drops to its coldest value the moment the sensor is unplugged | $50 to $250 installed |
| Coolant or water in the connector | Wet, green or crusted terminals; a coolant leak above or at the sensor | $20 to $200 |
| Damage from recent repair work | Code started after thermostat, water pump, head or intake work | $0 to $250 |
| Aftermarket gauge or add-on wired into the circuit | An aftermarket temperature gauge, adapter or engine monitor shares the sensor signal | $0 to $100 |
| Engine computer fault | Sensor and wiring test good and the reading is still wrong | $300 to $1,000 |
Stop driving if the engine is genuinely overheating: steam, a smell of hot coolant, a falling coolant level, or a boil-over at the reservoir. P0117 is usually a false hot reading, but while it is stored the temperature gauge and warning light may not be telling the truth in either direction, so check real signs rather than trusting the gauge. If the engine shows none of those signs, P0117 is safe to drive to a diagnosis, although cold starts may be rough.
What P0117 Actually Means
The engine coolant temperature (ECT) sensor is a thermistor, a resistor whose resistance falls as it gets hotter. The engine computer feeds it a reference voltage, normally 5 volts, through a resistor inside the computer, and reads the voltage left on the signal wire. A cold sensor has high resistance, so the signal voltage is high. A hot sensor has low resistance, so the voltage is low. The computer converts that voltage into a temperature, and it uses that temperature for almost everything: cold-start fuelling, idle speed, ignition timing, when to switch on the radiator fan, and, on many cars, the dashboard temperature gauge.
P0117 is “engine coolant temperature sensor 1 circuit low”. It sets when the signal voltage drops below anything a working sensor could produce. Volkswagen’s 2014 Beetle quick reference specification book shows both ways a manufacturer can express that. For the Beetle’s gasoline engines, P0117 sets when the calculated coolant temperature is above 140 °C (284 °F). For the 2.0L TDI diesel, the same book names P0117 “Engine Coolant Temperature Sensor Circuit Short to Ground” and sets it on a signal voltage below 0.15 volts. The same book defines P0118, the opposite code, as a temperature below −40 °C, or on the diesel a signal above 3.25 volts, “Open or Short to Battery Voltage”.
Why P0117 Is a Wiring Code, Not an Overheating Code
This is the point that saves money. A cooling system under pressure keeps coolant well below the temperatures that set P0117, and an engine that really did get that hot would be boiling over long before the code set. So a reading that high is the signature of a signal voltage being pulled toward zero, and the thing most likely to do that is a short to ground, either inside the sensor or somewhere along its signal wire.
Two free checks with a scan tool that shows live data will narrow it down before anything is bought:
- The cold-soak comparison. After the car has sat overnight, turn the ignition on without starting it and compare coolant temperature with intake air temperature. Both sensors have been sitting in the same cold engine, so they should read within a few degrees of each other and of the outside air. A coolant reading at or near the scan tool’s maximum on a stone-cold engine confirms a false reading.
- The unplug test. With the ignition on, unplug the coolant temperature sensor and watch the live reading. An open circuit should read as extremely cold, typically around −40 °F or −40 °C, which is the same number in both scales. If the reading drops to its coldest value, the short was inside the sensor or its connector, and the sensor is the likely part. If the reading stays pinned hot with the sensor unplugged, the short is in the harness between the connector and the computer, or in the computer itself, and a new sensor will not fix it.
Unplugging the sensor will set P0118 while you do it, which is expected. Clear the codes when you are done.
Symptoms You May Notice Alongside the Code
With the computer believing the engine is fully hot, it gives a cold engine the fuel mixture meant for a warm one. That usually means hard starting when cold, a rough or stalling idle until the engine warms up, and hesitation in the first few minutes of driving. Many vehicles also switch the radiator fan on and leave it running as a failsafe whenever the coolant temperature signal cannot be trusted. On cars where the gauge takes its reading from the same sensor, the gauge may sit at hot or behave oddly, and a high-temperature warning may appear. Once the engine is genuinely warm, it often runs close to normally. For warning-light behaviour more generally, see the site’s guide to why the coolant temperature light comes on.
1. Sensor Signal Wire Shorted to Ground
The sensor’s two wires run through the engine harness past hot components and brackets. Insulation hardened by heat can crack, and a harness out of its clips can rub through against metal. Once the signal wire touches ground, the voltage the computer reads collapses and it sees an impossibly hot engine. The unplug test above points here when the reading stays pinned hot with the sensor disconnected.
Harness faults can also take other circuits with them. Ford’s special service message 48991 for 2015 to 2020 vehicles with the 2.7L EcoBoost, including the F-150, Edge and Fusion, describes a check engine light and/or an engine coolant over-temperature warning with codes including P0116, P0117, P0118, P0119, P0128 and P0217, as well as knock sensor code P0330. It attributes these to the coolant temperature sensor or knock sensor wiring harness and corrects them by replacing both the coolant temperature sensor and the knock sensor. When P0117 arrives alongside codes from a sensor that shares its harness, the harness is the lead suspect.
To confirm it, disconnect both the sensor and the engine computer, then check the signal wire for continuity to ground. Any continuity is a short. Repair it with proper splices and heat-shrink, and re-secure the harness away from heat and sharp edges.
2. Failed Coolant Temperature Sensor
Sensors can fail with an internal short, which drops their resistance and reads as a very hot engine. The unplug test points here when the reading jumps to its coldest value as soon as the sensor is disconnected. To confirm, measure the sensor’s resistance across its two pins and compare it with the temperature-resistance chart in the service information for your engine. As a general pattern, a healthy sensor reads thousands of ohms when cold and a few hundred ohms when hot. A near-zero reading on a cold sensor condemns it.
Many engines have more than one coolant temperature sensor, sometimes one for the computer and a separate sender for the gauge, or two computer sensors in different parts of the cooling system. P0117 refers to sensor 1, so confirm which physical sensor that is for your engine before buying. The site’s article on the coolant temperature sensor’s purpose and location covers where it usually sits.
3. Coolant or Water in the Connector
The sensor is threaded into a coolant passage, so a seep past its seal, a leaking hose above it, or a weeping thermostat housing can wet the connector. Coolant is conductive enough to create a path between the terminals or to ground, and it leaves green or white crust as it dries. Unplug the connector and look at both halves. Clean or replace a contaminated connector, fix the leak that caused it, and replace the sensor’s sealing washer or O-ring if the leak was at the sensor itself.
4. Damage From Recent Repair Work
Coolant temperature sensors are often close to the thermostat housing, the water pump or under the intake manifold, so a thermostat, water pump, head gasket or intake job disturbs them. A connector cracked while being released, a harness pinched under a housing, or a pin pushed back in its socket can all set P0117 straight after the work. If the code appeared soon after a repair, start there. If the sensor was replaced, check it is the correct part number for your engine, because sensors with the same thread and plug can have different resistance curves.
5. Aftermarket Gauge or Add-On Wired Into the Circuit
Aftermarket temperature gauges, gauge adapters and engine monitoring devices are sometimes spliced into the factory sensor’s signal wire rather than using their own sender. Any extra load on that wire changes the voltage the computer reads, and a failing gauge or a poor splice can pull it low. If anything has been added, disconnect it and see whether the reading returns to normal.
6. Engine Computer Fault
Rare, and the last conclusion. If the reading stays pinned hot with the sensor unplugged and the signal wire shows no short to ground with both ends disconnected, the fault may be inside the computer. Check first for a service bulletin or software update, and remember that a new computer normally needs programming to the vehicle.
Diagnostic Order
- Check for real overheating first: coolant level, leaks, steam or boil-over. Do not dismiss a hot gauge as a false reading without looking.
- Record every stored code. Codes from sensors that share a harness point to the harness.
- Do the cold-soak comparison of coolant temperature against intake air temperature.
- Do the unplug test. Coldest value: sensor or connector. Stays hot: harness or computer.
- Inspect the connector for coolant, corrosion and damage, and remove any aftermarket add-on.
- Measure sensor resistance against the chart, or check the signal wire for a short to ground with both ends disconnected.
- Replace the sensor only once the test points at it.
After fitting a sensor, top up and bleed the cooling system, because removing a sensor from a coolant passage lets coolant out and can leave an air pocket. An air pocket around a new sensor can give erratic readings and a code of its own.
Repair Cost
Typical costs, labelled as typical because sensor location varies a great deal between engines:
- Coolant temperature sensor: roughly $50 to $250 installed, including a coolant top-up. Sensors buried under the intake manifold on some V engines cost more in labour.
- Connector cleaning or replacement: roughly $20 to $200.
- Wiring repair: roughly $50 to $300.
- Removing an aftermarket add-on: nothing to about $100.
- Engine computer: rarely needed; roughly $300 to $1,000 including programming.
Can You Keep Driving With a P0117 Code?
For a short time, carefully. The false hot reading usually makes cold starts rough and may keep the radiator fan running, but it rarely harms the engine by itself. The bigger risk is that the computer, and possibly the gauge, can no longer report the engine’s real temperature. If the car genuinely started to overheat while P0117 is stored, you might not get a trustworthy warning. Keep an eye on the coolant level, watch for steam or a hot-coolant smell, and fix the fault promptly.
If the gauge reads hot and there are real signs of overheating, stop. The site’s article on a flashing check engine light with an overheating engine covers what to do next.
Related Codes
- P0118: the opposite fault on the same sensor. See the P0118 guide. The signal voltage is too high, so the computer reads an impossibly cold engine, usually from an open circuit such as an unplugged connector or a broken wire.
- P0116: coolant temperature sensor range or performance. The signal is electrically plausible but does not behave as expected, for example it does not change as the engine warms.
- P0115 and P0119: a general circuit fault and an intermittent circuit fault on the same sensor.
- P0128: coolant temperature below the thermostat regulating temperature. It usually points to a thermostat stuck open rather than the sensor. See the P0128 guide and the signs of a bad thermostat.
- P0112: intake air temperature sensor circuit low. The same “impossibly hot” logic on a different sensor. P0112 and P0117 together suggest a shared fault such as a sensor ground or reference problem, or the computer.
Frequently Asked Questions
What does the P0117 code mean?
P0117 means the engine computer is reading a voltage from the coolant temperature sensor that is too low to be real. Because the sensor’s voltage falls as temperature rises, that tells the computer the engine is impossibly hot. It usually points to a short to ground in the sensor or its wiring, not a genuinely overheating engine.
Does P0117 mean my engine is overheating?
Usually not. P0117 sets on readings hotter than a pressurised cooling system normally reaches, so it is almost always a false reading. Still, check for real signs of overheating such as steam, coolant loss or a hot-coolant smell before assuming the gauge is wrong.
Is it safe to drive with a P0117 code?
For a short time, carefully. Cold starts may be rough and the radiator fan may run constantly, but the bigger risk is that the car can no longer warn you reliably if the engine really does overheat. Watch the coolant level and fix it promptly.
How do I know if it is the sensor or the wiring?
With the ignition on, unplug the sensor and watch the coolant temperature on a scan tool. If the reading drops to its coldest value, the short was in the sensor or its connector. If it stays pinned hot, the short is in the wiring or the computer.
What is the difference between P0117 and P0118?
They are opposites on the same sensor. P0117 means the signal voltage is too low, so the computer reads an impossibly hot engine, usually a short to ground. P0118 means the voltage is too high, so it reads an impossibly cold engine, usually an open circuit.
How much does it cost to fix P0117?
Roughly $50 to $250 installed for a coolant temperature sensor, $20 to $200 for a connector, and $50 to $300 for a wiring repair. An engine computer replacement is rarely needed.
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