# P0117: Engine Coolant Temperature Sensor 1 Circuit Low **Verdict:** Drive to a shop this week Cooling - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$400 - **Urgency:** This week - **Most likely:** Shorted ECT sensor or wiring - **DIY?:** Steps 1-3 ## Quick answer P0117 means the voltage from the engine coolant temperature (ECT) sensor is too low, which the engine computer reads as an extremely hot engine. Usually the sensor or its wiring is shorted and the engine is not really overheating, but check the real temperature first. Expect the cooling fans to run nonstop and possibly hard cold starts. ## What does P0117 mean? The ECT sensor is a thermistor screwed into a coolant passage, usually near the thermostat housing or in the cylinder head. Its resistance falls as the coolant heats up. The engine computer sends it a reference voltage, usually 5 volts, and reads the voltage left on the signal wire, which drops as the engine warms. Coolant temperature affects cold-start fuel, idle speed, spark timing, fan control, shifting and when other monitors may run. P0117 sets when the signal voltage falls below a calibrated minimum, often a few tenths of a volt, which a scan tool shows as an impossibly high temperature, far above the boiling point of coolant. Many makers set it within seconds. The computer then assumes a default temperature, and on many vehicles runs the cooling fans full time as a safeguard. "Sensor 1" is the main ECT; some engines have a second one. P0118 is the opposite, high-voltage fault. ## What are the symptoms of P0117? - Check engine light on - Cooling fans running constantly, even on a cold engine - Hard starting or stalling when the engine is cold - Temperature gauge reading hot on cars where it shares this sensor - A/C may cut out on some vehicles as an overheat precaution ## What causes P0117? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Shorted coolant temperature sensor** (most common). After years of heat cycles, the thermistor inside the sensor can short and drop the signal toward zero. Coolant seeping into the sensor body speeds this up, especially with old or incorrect coolant. If unplugging the connector makes the scan tool reading jump to about -40 degrees, the wiring is fine and the sensor itself is shorted. 2. **Signal wire shorted to ground** (common). The ECT harness runs close to hot engine parts, so its insulation can bake, crack and touch ground. Damage near the thermostat housing, spots rubbed against brackets and rodent bites are typical. If the reading stays hot with the sensor unplugged, the signal wire is shorted to ground somewhere between the sensor and the computer. 3. **Coolant or water in the connector** (less common). A leaking thermostat housing, heater hose or the sensor's own seal can drip coolant into the connector. Coolant conducts electricity and can create a path from the signal pin to ground. Green or white crust on the terminals is the giveaway. Fix the leak, clean or replace the connector, and renew the sensor seal. 4. **Wrong replacement sensor** (rare). Many engines use sensors that look alike but have different resistance curves or pin layouts, and a gauge sender can resemble the computer's sensor. A wrong part may thread in and plug in yet read far enough off to set a circuit code. If P0117 appeared right after a sensor swap, compare the part number with the correct one. ## How do you diagnose P0117? 1. First confirm the engine is not actually overheating: check the coolant level with the engine cold and watch for steam or coolant loss. 2. Read ECT on a scan tool after the car has sat overnight; it should closely match outside air and the IAT reading. 3. Unplug the sensor; if the reading drops to about -40 degrees, the sensor is shorted and should be replaced. 4. If it still reads hot with the sensor unplugged, trace the signal wire for a short to ground. 5. Inspect the connector for coolant and corrosion, and fix any leak above it. ## How do you fix P0117, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace coolant temperature sensor | $100-$350 | Higher end on engines where the sensor sits under the intake manifold. | | Repair wiring or connector | $100-$400 | For a chafed signal wire or a coolant-soaked connector. | | Fix a thermostat housing leak that soaked the connector | $150-$400 | Gasket or housing replacement plus a coolant top-up. | Full price breakdowns: [thermostat replacement cost](/cost/thermostat-replacement), [coolant flush cost](/cost/coolant-flush). ## Is it safe to drive with P0117? > **Drive to a shop this week.** If the engine is not truly overheating, you can usually drive short distances while you arrange the repair. Cold starts may be hard and the fans will run constantly. If the gauge reads hot along with steam, coolant loss or a sweet smell, stop driving and let the engine cool. ## Will P0117 fail an emissions test? P0117 keeps the check engine light on, which fails an OBD-II emissions test in most states. Many monitors wait for a confirmed warm engine before they run, so a bad coolant temperature signal can also keep readiness monitors from completing. After the repair, drive several days of mixed trips, including full warm-ups, before testing. ## After the repair: clearing P0117 Clear the code and watch ECT on a scan tool from a cold start. It should begin near outside temperature, rise steadily and level off near the thermostat's rated temperature, often somewhere around 190 to 220 degrees F. The cooling fans should cycle instead of running nonstop, and cold starts should be normal again. ## How to prevent P0117 - Change coolant at the interval in the owner's manual, using the type the maker specifies. - Fix small coolant leaks near the thermostat housing before they reach sensor connectors. - Use only the ECT sensor listed for your exact engine; similar-looking parts can read differently. - Keep wiring clipped away from exhaust manifolds and hot engine surfaces after any repair. ## Codes often seen with P0117 - [P0118](/codes/p0118): Engine Coolant Temperature Sensor 1 Circuit High - [P0128](/codes/p0128): Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) - [P0217](/codes/p0217): Engine Coolant Over Temperature Condition - [P0112](/codes/p0112): Intake Air Temperature Sensor 1 Circuit Low (Bank 1) - [P0125](/codes/p0125): Insufficient Coolant Temperature for Closed Loop Fuel Control ## Frequently asked questions ### Can I drive with a P0117 code? You can usually drive short distances with P0117 if the engine is not really overheating. Check the coolant level and the temperature gauge first. Expect the cooling fans to run nonstop and cold starts to be harder. Stop driving immediately if there is steam, coolant loss or a sweet smell. ### Does P0117 mean my engine is overheating? Usually not. P0117 means the coolant temperature sensor circuit reads a lower voltage than any real temperature would produce, which looks like extreme heat. Most often the sensor or its wiring is shorted. Still, confirm the real temperature by checking the coolant level when cold and watching for steam before blaming the sensor. ### Why are my cooling fans running all the time with P0117? Many engine computers run the cooling fans at full speed whenever the coolant temperature signal is invalid. It is a safety backup, because the computer cannot tell how hot the engine really is. The fans should return to normal cycling once the sensor or wiring is fixed and the code is cleared. ### How much does it cost to fix P0117? Most P0117 repairs cost $100 to $400 at an independent shop. A new coolant temperature sensor usually costs $100 to $350 installed, more on engines where it sits under the intake manifold. Wiring or connector repair runs $100 to $400, and fixing a leak that soaked the connector costs $150 to $400. ### Can I replace a coolant temperature sensor myself? Yes, on many cars replacing the ECT sensor is a moderate home job. Let the engine cool completely, have matching coolant ready and use a new seal on the sensor. Refill and bleed air from the system afterward. On some V engines the sensor is buried under the intake, which makes it a shop job. ### What is the difference between P0117 and P0118? P0117 means the coolant temperature sensor signal voltage is too low, which reads as an extremely hot engine and usually points to a short. P0118 means the voltage is too high, which reads as an extremely cold engine and usually points to an open circuit, such as an unplugged connector or a broken wire. ### Can P0117 cause hard starting? Yes, P0117 can cause hard starting, mostly when the engine is cold. The computer thinks the engine is already hot, or uses a default value, so it may not add the extra fuel a cold engine needs. The engine may crank longer, stumble or stall until it warms up. ### Can low coolant cause P0117? Low coolant does not usually cause P0117 by itself, because a sensor sitting in air or steam does not short the circuit. Low coolant more often causes erratic readings or real overheating. Check the level anyway, since the same leak that drops the coolant can drip into the sensor connector and short it. ### Will P0117 affect my temperature gauge? It can. On many vehicles the dashboard gauge uses the same coolant temperature sensor, so it may read hot or sit at maximum. Other vehicles use a separate gauge sender, so the gauge stays normal while P0117 is stored. Trust a confirmed real temperature over either reading. ### Can a bad thermostat cause P0117? No, a bad thermostat does not usually cause P0117. A thermostat stuck open tends to set P0128, and one stuck closed causes real overheating. P0117 is an electrical fault in the sensor circuit. A leaking thermostat housing can, however, wet the nearby sensor connector and cause a short. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) - [Car Care Council: Car care guide](https://www.carcare.org/car-care-guide/) ## Other cooling codes - [P0106 Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance](https://diagxcar.com/codes/p0106) - [P0107 Manifold Absolute Pressure/Barometric Pressure Circuit Low](https://diagxcar.com/codes/p0107) - [P0108 Manifold Absolute Pressure/Barometric Pressure Circuit High](https://diagxcar.com/codes/p0108) - [P0112 Intake Air Temperature Sensor 1 Circuit Low (Bank 1)](https://diagxcar.com/codes/p0112) - [P0113 Intake Air Temperature Sensor 1 Circuit High (Bank 1)](https://diagxcar.com/codes/p0113) - [P0118 Engine Coolant Temperature Sensor 1 Circuit High](https://diagxcar.com/codes/p0118) - [P0121 Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0121) - [P0122 Throttle/Pedal Position Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0122) - [P0123 Throttle/Pedal Position Sensor/Switch "A" Circuit High](https://diagxcar.com/codes/p0123) - [P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control](https://diagxcar.com/codes/p0125) ## Symptoms - [Why is my car overheating?](https://diagxcar.com/symptoms/car-overheating) - [Why does my car crank but not start?](https://diagxcar.com/symptoms/car-cranks-but-wont-start) - [Why does my car stall while driving?](https://diagxcar.com/symptoms/car-stalls-while-driving) - [Why is black smoke coming from my exhaust?](https://diagxcar.com/symptoms/black-smoke-from-exhaust) ## Repair costs - [Thermostat replacement cost](https://diagxcar.com/cost/thermostat-replacement): $150-$450 - [Coolant flush cost](https://diagxcar.com/cost/coolant-flush): $110-$260 --- Source page: https://diagxcar.com/codes/p0117 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.