P0117 — Engine Coolant Temperature Sensor Circuit 1 Low Input

P0117 — Engine Coolant Temperature Sensor Circuit 1 Low Input. A fault in the engine cooling and temperature control. Short, gentle driving to a shop is usually possible. Typical causes: low coolant level or air in the system, failed cooling fan or fan relay, water pump or radiator issues.

powertrain (engine/transmission)

Is it safe to drive?

Monitor the temperature closely P0117 can make the gauge or ECU believe the engine is extremely hot. Driving may be possible if the temperature remains stable, but stop immediately if the gauge climbs, a coolant-temperature warning appears, coolant is low, or the engine shows signs of actual overheating.

Typical symptoms

Temperature gauge reads excessively high or remains near maximum
Cooling fans run continuously or at high speed even when the engine is only warm
Check-engine light is on with P0117 stored in the powertrain controller
OBD live data shows an engine coolant temperature far above the actual temperature
Engine may run rough, produce elevated fuel trims, or use a fail-safe temperature value

Common causes

Engine coolant temperature sensor with an internal short to ground
Coolant temperature sensor signal Circuit 1 shorted to ground by damaged wiring
Corroded or damaged ECT connector allowing the signal terminal to short or leak current
Low or unstable sensor reference voltage, commonly below the expected 5-volt supply
High-resistance or open ground connection that biases the P0117 sensor signal low

How to diagnose with an OBD scanner

Confirm the false temperature with live data Using an OBD scanner through an ELM327 adapter or AIScanAuto, read the engine coolant temperature PID and P0117 freeze-frame data. After a full cool-down, compare the key-on ECT value with actual ambient temperature; a near-hot reading on a cold engine supports a low-input circuit fault.
Inspect the ECT connector and wiring Examine the engine coolant temperature sensor connector, pins, and harness for coolant contamination, green corrosion, trapped water, crushed wires, chafing, or pin damage. Check whether aftermarket work or a previous repair left Circuit 1 incorrectly connected.
Test reference voltage and ground Back-probe the ECT connector with a multimeter. Verify that the reference supply is near the specified value, typically 5 volts, and that the ground side has a negligible voltage drop. A low reference or unstable ground must be traced through the relevant fuse, wiring, and connector pins.
Separate sensor failure from a wiring short Disconnect the ECT sensor and measure its resistance across the specified terminals, comparing the result with the manufacturer temperature curve. A near-zero or fixed resistance indicates a shorted sensor; if sensor resistance is normal, test Circuit 1 for continuity to ground and inspect the connector.
Repair and verify P0117 Replace the failed sensor or repair the defective reference, ground, connector, or signal wiring. Clear P0117 with the scanner, restart the vehicle, and confirm that the ECT reading approaches actual coolant temperature and that the code does not return.

Approximate repair cost

A typical independent-shop estimate is $40-$120 for the ECT sensor plus about $90-$250 in diagnostic and replacement labor, or roughly $130-$370 total. Wiring or connector repair can raise the total to about $400. Figures are estimates and vary by region and vehicle.

Common questions

Can I drive with P0117 stored?

Yes, but cautiously. P0117 can make the gauge or ECU believe the engine is extremely hot, so monitor the gauge and coolant warning and stop if genuine overheating or low coolant is found.

Why does P0117 commonly appear when the engine is cold?

P0117 means the Circuit 1 voltage is below the ECU's expected range. A shorted sensor, signal wire shorted to ground, damaged connector, or low reference supply can create this condition.

Should I replace the coolant temperature sensor first for P0117?

Not automatically. Check the connector, reference voltage, ground, and signal wiring first; replace the ECT sensor only if its resistance does not match the specified temperature curve.

Will clearing P0117 with a scanner make the problem go away?

No. Clearing P0117 only erases the stored fault; if the low-input condition remains, the code will generally return during the next drive cycle.

When the engine control module detects that the signal from the engine coolant temperature sensor has dropped below its normal operating range, it stores diagnostic trouble code P0117. This code specifically indicates a low input condition on circuit one of the sensor circuit — essentially, the voltage the module receives from the sensor is stuck near ground when it should be reading a value that corresponds to the actual coolant temperature. The module sets this code after monitoring the sensor for a set number of drive cycles and confirming the reading stays outside acceptable bounds.

The engine coolant temperature sensor plays a straightforward but critical role: it is a thermistor that changes resistance based on coolant temperature, and the engine control module uses this information to adjust fuel delivery, fan operation, and transmission shift points. When the sensor circuit experiences a short to ground, a failed sensor, or a wiring problem, the module loses accurate temperature data and defaults to a failsafe mode. The freeze frame data saved alongside P0117 captures engine speed, load, coolant temperature, and vehicle speed at the moment the fault was detected — invaluable information for reproducing the failure condition during diagnosis.

Symptoms of P0117

P0117 typically produces a recognizable cluster of symptoms that reflect the module's attempts to compensate for missing or incorrect temperature data:

The Check Engine Light or malfunction indicator lamp illuminates on the dashboard
The temperature gauge reads abnormally low or fails to climb toward the normal operating range
Cabin heating performance suffers because the module does not recognize that the engine needs heat
The cooling fans may run continuously or fail to activate at the correct temperature
Fuel consumption typically increases as the engine control module relies on default enrichment values
In some cases, the engine may run rougher than normal or exhibit hesitation under load

Common Causes of P0117

The root cause of P0117 almost always involves either the sensor itself, its wiring, or the voltage supply feeding the circuit. Most likely culprits are listed first:

A failed engine coolant temperature sensor that drags the signal voltage down
Wiring damage — a chafed wire, pinched harness, or corroded connector that creates a short to ground
A broken or interrupted supply voltage path to the sensor circuit
Low coolant level or air trapped in the cooling system causing erratic sensor readings
A faulty cooling fan relay or failed fan motor affecting related circuit behavior
Water pump or radiator issues that alter coolant flow and temperature profile
Problems elsewhere in the engine wiring harness that share the same ground or signal path

How to Diagnose P0117

A systematic approach prevents unnecessary parts replacement and gets to the root cause efficiently:

Connect an OBD2 scanner, retrieve P0117, and record the freeze frame data — this shows RPM, load, coolant temperature, and vehicle speed at the moment the fault occurred and guides the next steps.
Scan for any other stored trouble codes; related codes in the same circuit or affecting the cooling system often point toward the real culprit.
Visually inspect the engine coolant temperature sensor wiring harness and connector for damaged insulation, broken strands, corrosion, or loose terminals — the fault is frequently in the connector.
Measure voltage at the sensor connector with the key on and engine off; then check resistance from the signal wire to ground — a reading near zero ohms confirms a short to ground somewhere in the harness.
Monitor live data from the coolant temperature sensor during a warm-up cycle; it should rise smoothly and reach the expected operating temperature range within a predictable time frame.
Verify coolant level and inspect the upper and lower radiator hoses for proper heatup — an obviously cold lower hose suggests the thermostat is stuck open, which can contribute to the sensor reading staying low.
Clear P0117, perform a drive cycle that reproduces the freeze frame conditions, and verify the code does not return — a one-time event may indicate an intermittent connection rather than a hard failure.

Repair Options and Costs

P0117 is usually repairable without major expense unless the root cause involves the radiator, water pump, or an extensive wiring harness issue. Common repair paths with typical parts and labor costs at independent shops in the United States:

Replace the engine coolant temperature sensor: $100–$300
Replace the thermostat housing or thermostat: $150–$400
Repair the wiring harness or connector: labor costs vary widely depending on access; wiring repair kits and connectors run $20–$80 plus labor
Full cooling system work such as water pump, radiator, or fan replacement: $300–$1,200 depending on the component and vehicle

The total cost to resolve P0117 generally falls between $100 and $1,200. Spending time on a careful diagnosis before purchasing parts almost always saves money — the most expensive repair on the list is rarely where the problem actually lies.

Can You Drive with P0117?

Drive with caution when P0117 is active. The vehicle will continue operating, but the engine control module substitutes default values for coolant temperature, which typically leads to richer fuel mixtures, poor cabin heat, and erratic fan behavior. A thermostat stuck open mainly burns extra fuel; however, if the temperature gauge begins climbing toward the red zone or you notice steam or coolant loss, stop immediately — continuing to drive an overheating engine risks catastrophic head gasket failure or permanent engine damage.