Your scan tool might show a coolant temperature near 300°F while the engine is cold. The cooling fans may roar the moment you start the car. A P0117 code means the engine coolant temperature sensor signal dropped below its lowest allowed voltage.
This guide explains why a low voltage reads as a hot engine, what causes it and how to test the circuit.
P0117 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 17 | Specific fault | Engine Coolant Temperature Circuit Low Input |
What the P0117 Code Means for Your Engine
The SAE J2012 standard defines the P0117 code as Engine Coolant Temperature Circuit Low Input. “Low input” refers to the signal voltage.
The engine coolant temperature (ECT) sensor is a thermistor whose resistance drops as coolant heats up. The powertrain control module (PCM) sends it a 5-volt reference through a resistor. Hot coolant pulls the voltage down. A signal near 0 volts tells the PCM the coolant is extremely hot, often around 300°F on the scan tool.
Real coolant would boil over long before reaching that number. So the PCM treats the reading as a fault, usually a short to ground, and switches to a default temperature.
Common Symptoms
A false hot reading changes how the engine and cooling system behave. Symptoms of a P0117 code include:
- Check engine light on
- Cooling fans running constantly
- Hard starting when the engine is cold, because the PCM skips its cold-start enrichment
- Rough idle or stalling during warm-up
- Temperature gauge reading high on vehicles that share the sensor
What Causes the P0117 Code?
Low input faults are almost always electrical. These are the usual causes:
- Signal wire shorted to ground. Insulation melted or rubbed through near the engine.
- Shorted ECT sensor. The thermistor fails internally and drops to near zero resistance.
- Coolant in the connector. A leaking sensor seal can bridge the pins.
- PCM fault. A shorted PCM input is rare.
A real overheating engine does not set this code. A true overheat shows a high but believable temperature, not a reading at the sensor’s limit.
How to Test the Circuit
You need a digital multimeter, a scan tool and the wiring diagram for your engine. Make sure the engine is cold before you work near the cooling system.
Use the Unplug Test
Turn the key on and note the ECT reading. Then unplug the sensor. The scan tool should drop to about -40°F, the coldest value it can show.
If the reading drops, the wiring is good, and the sensor has shorted inside. If the reading stays hot with the sensor unplugged, the signal wire is shorted to ground between the connector and the PCM.
Measure the Sensor and Wiring
Measure the sensor’s resistance across its two pins. A reading near 0 ohms confirms an internal short. Many ECT sensors read about 2,000 to 3,500 ohms at 68°F, so compare yours with the service manual chart.
To check the wiring, disconnect the battery and unplug the sensor and the PCM connector if reachable. Measure from the signal wire to engine ground. Any continuity means the wire is shorted.
Fixes That Clear the P0117 Code
Fix the fault your tests found, then clear the code and drive the vehicle. The most common repairs are:
- Repair the shorted wire with solder and heat shrink tubing, and route it away from hot parts.
- Clean and dry the connector, then seal it with dielectric grease.
- Replace the ECT sensor with the engine cold, and top off any coolant you lose.
- Fix the coolant leak at the sensor seal if coolant reached the connector.
Typical Repair Costs
Repairs for the P0117 code are usually affordable. This table shows common ranges.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Connector cleaning or pigtail | $10 to $40 | $50 to $200 |
| Wiring repair | $10 to $30 | $100 to $300 |
| ECT sensor replacement | $15 to $60 | $100 to $250 |
| Coolant top-off | $15 to $40 | $50 to $100 |
Codes That Often Appear Together
Low coolant temperature voltage codes often come with related faults. These codes help you narrow the search:
- P0115: General ECT circuit malfunction
- P0116: ECT reading out of the expected range
- P0118: ECT signal too high, which reads as a very cold engine
- P0112: IAT sensor low input, which can point to a shared wiring fault
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0114 | Intake Air Temperature Circuit Intermittent |
| P0115 | Engine Coolant Temperature Circuit Malfunction |
| P0116 | Engine Coolant Temperature Circuit Range/Performance Problem |
| P0118 | Engine Coolant Temperature Circuit High Input |
| P0119 | Engine Coolant Temperature Circuit Intermittent |
Bringing a Low Coolant Temperature Signal Back to Normal
A P0117 code means the PCM sees a voltage that reads as impossibly hot coolant. A short to ground or a shorted sensor causes nearly every case.
- Note the ECT reading with the key on.
- Unplug the sensor and confirm the reading drops to -40°F.
- Measure sensor resistance and check the wire for shorts.
- Replace the sensor only after the wiring tests good.
Frequently Asked Questions
Is My Engine Really Overheating?
A reading near 300°F with this code is almost never real. Check the temperature gauge, the coolant level and the upper radiator hose. If the hose is only warm and the coolant is not boiling, the reading is false. If you see steam or the gauge climbs fast, stop and let the engine cool.
Is It Safe to Drive with a Low ECT Signal?
Short trips are usually possible, since the PCM runs the fans and uses a default temperature. Cold starts may be hard because the PCM thinks the engine is hot. Watch the gauge closely, since you may not see a real overheat. Get the circuit fixed soon.
Why Won’t My Car Start When Cold?
A cold engine needs extra fuel to start. When the sensor reads hot, the PCM skips that extra fuel, so the engine cranks long or stalls. Pressing the gas pedal slightly while cranking may help as a short-term workaround. The real fix is repairing the sensor circuit.
Can the Wrong Coolant Damage the Sensor?
Mixing incompatible coolants or using plain water can lead to corrosion and deposits on the sensor tip. Over time, that can make the sensor read wrong or leak at its seal. Use the coolant type your owner’s manual lists, and flush the system at the recommended interval.