P0117 Trouble Code: Meaning, Causes, Symptoms & Fixes

One-line meaning: P0117 means the engine coolant temperature sensor 1 circuit is sending abnormally low voltage to the computer, which reads it as an extremely high coolant temperature.

What the P0117 Code Means

The P0117 code is one of the most misunderstood check engine light codes drivers search for. Its full definition is “Engine Coolant Temperature Sensor 1 Circuit Low.” In plain English, your car’s computer (ECM/PCM) has detected that the voltage signal coming from the primary coolant temperature sensor has dropped below its normal operating range.

Here is the part that trips everyone up: “low” refers to the electrical signal, not the temperature. The coolant temperature sensor is a thermistor whose resistance falls as it gets hotter, so low voltage tells the computer the coolant is extremely hot, often interpreted as 250°F or more. If your scanner shows the p0117 engine code and your temperature gauge is pinned to hot on a cold engine, that false overheating reading is exactly what this code describes.

This is the opposite of a minus 40 degree reading. A minus 40°F reading means the circuit is open, from a broken wire or an unplugged sensor, and that sets P0118, not P0117. P0117 means a short to ground or a failed sensor is pulling the signal voltage down. Keep that distinction in mind, because it changes the entire diagnosis.

Why does one sensor matter so much? The computer bases fuel delivery, ignition timing, cooling fan operation, and even the dashboard temperature gauge on this single reading. When the reading is wrong, every one of those systems makes wrong decisions at the same time. That is why a small, inexpensive sensor can cause such dramatic symptoms.

“Sensor 1” is the primary engine coolant temperature sensor, usually threaded into the thermostat housing or cylinder head. Some vehicles have a second sensor at the radiator outlet, but P0117 refers specifically to sensor 1.

Symptoms of a P0117 Code

In many cases, the check engine light is the first and only obvious symptom. But because the computer believes the engine is overheating, you may notice several of these:

• Check engine light on with P0117 stored
• Temperature gauge reading hot, or an overheating warning, on a cold engine
• Cooling fans running constantly at high speed
• Poor fuel economy from corrupted fuel trim
• Rough idle, hesitation, or hard starting, especially on cold starts when the computer skips the cold-start enrichment the engine actually needs
• Cabin heater blowing lukewarm air
• In severe cases, the computer may limit performance to protect the engine

The hallmark symptom is the mismatch: a gauge screaming “hot” while the engine is clearly cold to the touch. If you see that, the sensor circuit is lying to the computer, not the cooling system overheating.

Common Causes of a P0117 Code

P0117 is an electrical fault first and a cooling system fault second. Here are the causes, roughly in order of likelihood:

• Short to ground in the sensor wiring: chafed or melted insulation lets the signal wire touch ground, pulling the voltage to near zero. This is the classic P0117 cause.
• Failed coolant temperature sensor: the thermistor inside the sensor shorts internally and stops responding to temperature changes.
• Corroded or loose connector: moisture, coolant, or oil in the sensor plug corrupts the signal.
• Low coolant level or trapped air: if the sensor tip is not fully submerged in coolant, its readings turn erratic.
• Stuck-open thermostat: a thermostat that never closes keeps the engine from warming up and can confuse temperature diagnostics.
• Faulty engine computer: rare, but possible after every other cause has been ruled out.

How to Diagnose a P0117 Code (Do This Before Buying Parts)

P0117 is often misdiagnosed as a bad sensor when the real problem is a damaged wire. Work through this order before spending money:

1. Scan and document. Connect an OBD-II scanner, confirm P0117, and check the freeze-frame data. Look at the live coolant temperature reading: if it shows an implausibly high value such as 250°F on a cold engine, the circuit is confirmed as the problem, not actual overheating.
2. Check the coolant level. With the engine fully cold, verify the reservoir sits between MIN and MAX. Top up with the correct coolant if it is low, and look for leaks.
3. Inspect the wiring and connector. Unplug the ECT sensor and look for corrosion, bent pins, oil or coolant contamination, and any wiring with cracked, melted, or chafed insulation that could short to ground.
4. Test the sensor. With the sensor disconnected and the engine cold, measure resistance across its terminals with a multimeter. Expect roughly 2 to 3 kΩ at around 20°C (68°F). A reading near zero means the sensor is shorted internally.
5. Check the reference signal. With the connector unplugged and the key on, verify the reference voltage at the harness side, and check the signal wire for continuity and shorts to ground.
6. Clear and verify. After the repair, clear the code, drive through a full warm-up cycle, and confirm P0117 does not return.

How to Fix a P0117 Code

The right fix depends on what your diagnosis found:

• Replace the coolant temperature sensor: drain a little coolant, unscrew the old sensor, thread in the new one to the specified torque (commonly 15 to 20 Nm), and reconnect the plug.
• Repair damaged wiring: cut out the damaged section, splice and crimp, seal with heat shrink, and confirm continuity with no short to ground.
• Clean or replace the connector: remove corrosion, make sure the pins grip tightly, and fit a pigtail replacement if the plug itself is damaged.
• Top up and bleed the cooling system: refill with the correct 50/50 coolant mix to the MAX line and bleed any trapped air.
• Replace a stuck-open thermostat: only if testing proves it is actually faulty.
• Clear the code and test drive: erase the code with your scanner and verify the temperature reading behaves normally through a full drive cycle.

P0117 Repair Costs

Drivers always ask how much it costs to fix P0117. The sensor itself is cheap; labor is what varies. Here are realistic ranges:

The sensor is a $15 to $50 part on most vehicles, so this is one of the cheaper check engine light codes to fix. Costs climb only when damaged wiring is buried deep in the harness.

DIY vs Mechanic Verdict

You can do this at home: checking the coolant level, inspecting the sensor connector and visible wiring, replacing the coolant temperature sensor with basic hand tools, and topping up and bleeding the coolant.

See a mechanic for: wiring shorts hidden inside the harness, intermittent faults that need an oscilloscope, any signs of real overheating damage, or if you are not comfortable working around the cooling system.

Bottom line: P0117 is one of the most DIY-friendly codes. The part is inexpensive, the sensor is usually easy to reach, and the diagnosis is straightforward with a basic scanner and multimeter.

Can You Drive With a P0117 Code?

For short trips, usually yes, but do not ignore it for long. The computer thinks the engine is overheating, so it may run the cooling fans nonstop and alter the fuel mixture, which hurts fuel economy and drivability.

The bigger risk is blindness: with the temperature signal corrupted, you have no reliable warning if the engine starts genuinely overheating. If you see steam, smell hot coolant, or the engine feels abnormally hot, stop driving immediately. Otherwise, get it diagnosed within a few days.

P0117 on Popular Vehicles

P0117 is a generic code with the same meaning on every brand, but sensor location and common failure points vary:

• p0117 honda: on Honda Civic and Accord models, the primary ECT sensor sits at the thermostat housing, and corroded connectors are a frequent culprit.
• p0117 toyota: on Toyota Corolla and Camry models, check the sensor near the thermostat and the wiring where it routes past hot exhaust parts.
• p0117 ford: on Ford F-150 and Focus models, chafed wiring near the engine is a common source of the short to ground.
• p0117 nissan: on Nissan Altima and Sentra models, the sensor is accessible at the coolant outlet, and air pockets after a coolant service can trigger false readings.

On European cars that also follow EOBD, the same P0117 definition applies, since P0xxx codes are standardized across all markets.

P0117 vs P0118 and P0116

These three codes are constantly confused, so here is the difference. P0117 means the sensor circuit voltage is too LOW, which the computer reads as extremely HOT, usually from a short to ground or a shorted sensor. P0118 means the circuit voltage is too HIGH, which the computer reads as minus 40 degrees, usually from an open circuit, a broken wire, or an unplugged sensor. They are electrical mirror images of each other.

P0116 is different again: it means the sensor reading is not changing the way the computer expects, a range or performance problem often caused by a stuck thermostat rather than an electrical fault. If you have P0117, chase the wiring and the sensor, not the thermostat, unless your testing points there.

Related Codes

Related codes include P0116, P0118, P0119, P0128, P0480, P0481, and P2181.

Video Walkthrough

Watch this quick walkthrough from Car Mechanic USA showing how the P0117 code is diagnosed and fixed, including the sensor location and the wiring checks that catch the short to ground.

P0117 FAQs

Conclusion

A P0117 code looks alarming because the computer insists the engine is overheating, but the fix is usually simple: a shorted wire, a failed sensor, or low coolant. Work through the diagnosis in order, fix what you find, and verify with a test drive. Your temperature gauge will thank you.

About the author: Written by the OBD2 Code Hub editorial team. Our diagnostic guides are built from real OBD-II data, manufacturer service procedures, and hands-on troubleshooting experience, then reviewed for technical accuracy before publishing. We drive the same cars you do.