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P0117 Code Explained: Coolant Temperature Sensor Circuit Low Input

P0117 Code

If your Check Engine Light is on and your OBD-II scanner displays P0117, the Engine Control Module has detected an abnormally low-voltage signal from the Engine Coolant Temperature sensor circuit.

Despite the wording, P0117 does not always mean the engine is actually overheating. It means the ECM is receiving an electrical signal that suggests the coolant temperature is extremely high—sometimes beyond the realistic operating range of the engine.

The most common causes include a failed coolant temperature sensor, damaged wiring, a short to ground, connector contamination, or an actual overheating condition.

Because the Engine Coolant Temperature sensor affects fuel delivery, ignition timing, radiator fan operation, idle speed, and emissions control, P0117 should be diagnosed promptly.

Quick Answer

ItemInformation
CodeP0117
DescriptionEngine Coolant Temperature Sensor Circuit Low Input
Severity⭐⭐⭐⭐☆ High
Safe to Drive?Only after confirming the engine is not overheating
Most Common CausesFailed ECT sensor, shorted wiring, overheating
Estimated Repair Cost$20–$1,500+

What Does the P0117 Code Mean?

Diagnostic Trouble Code P0117 means the ECM has detected voltage from the Engine Coolant Temperature sensor circuit that is below the manufacturer’s acceptable range.

The ECT sensor is usually a thermistor whose resistance changes with temperature.

Generally:

When circuit voltage becomes extremely low, the ECM may interpret the signal as an excessively hot engine.

Common reasons include:

What Does the Engine Coolant Temperature Sensor Do?

The Engine Coolant Temperature sensor monitors the temperature of coolant circulating through the engine.

The ECM uses this information to control:

When the sensor indicates that the engine is hot, the ECM may reduce fuel enrichment, activate the cooling fans, alter ignition timing, or enter a protective operating mode.

What Does “Circuit Low Input” Mean?

The phrase circuit low input refers to electrical voltage—not necessarily low coolant temperature.

A low signal voltage tells the ECM that sensor resistance is very low.

The ECM may interpret this as:

On some scan tools, coolant temperature may display an unrealistic value such as 280°F to 300°F or higher.

That does not automatically mean the engine has achieved lava status. It means the sensor circuit needs to be tested before the engine is accused of attempting atmospheric reentry.

Where Is the Coolant Temperature Sensor Located?

The ECT sensor is commonly installed near:

Some vehicles use multiple temperature sensors.

One may send data to the ECM, while another controls the dashboard gauge. Other vehicles use a single dual-function sensor.

Confirm the correct sensor location with a service manual or wiring diagram before replacing parts.

Symptoms of a P0117 Code

Common symptoms include:

If the engine is genuinely overheating, symptoms may also include:

How Serious Is P0117?

Severity: ⭐⭐⭐⭐☆ High

P0117 can be caused by a simple electrical problem, but it can also appear during an actual overheating condition.

Potential consequences include:

The first priority is confirming whether the engine is actually overheating.

Can You Drive With P0117?

Do not continue driving until you verify the engine’s actual temperature.

You may be able to drive a short distance if:

Stop driving immediately if:

Driving an overheating engine is an excellent way to turn a small sensor problem into a full engine replacement. Mechanics love this one weird trick. Your wallet does not.

Most Common Causes of P0117

CauseLikelihood
Failed coolant temperature sensor⭐⭐⭐⭐⭐
Signal wire shorted to ground⭐⭐⭐⭐
Corroded connector⭐⭐⭐⭐
Coolant contamination in connector⭐⭐⭐
Actual engine overheating⭐⭐⭐
Damaged wiring harness⭐⭐⭐
Low coolant level⭐⭐
Failed thermostat⭐⭐
Cooling fan failure⭐⭐
Failed ECM⭐

Electrical faults are common, but actual cooling-system problems must always be ruled out first.

Most Common Repairs

RepairTypical Cost
Reconnect or clean sensor connectorFree–$75
Replace coolant temperature sensor$50–$250
Repair wiring$100–$400
Replace damaged connector$75–$300
Cooling-system repair$100–$1,500+
Replace thermostat$150–$500
Replace cooling fan assembly$300–$900
ECM repair or replacement$800–$2,000+

Repair cost depends heavily on whether the problem is electrical or caused by actual overheating.

Tools You May Need

Diagnosing P0117 may require:

Step-by-Step P0117 Diagnosis

Step 1: Check for Actual Overheating

Before testing the sensor circuit, inspect the cooling system.

Check:

Never remove the radiator cap while the engine is hot.

Hot pressurized coolant does not care that you watched a tutorial first.

Step 2: Compare Scan Data With Actual Temperature

Use a scan tool to observe Engine Coolant Temperature.

Compare the reading with:

After sitting overnight, coolant temperature and intake air temperature should be reasonably close.

If ambient temperature is 70°F but the scan tool reports 290°F before startup, the sensor circuit is clearly inaccurate.

Step 3: Read Freeze-Frame Data

Record:

Freeze-frame data can show whether P0117 occurred:

Step 4: Inspect the ECT Sensor Connector

Check the connector for:

Coolant can travel through a damaged sensor and contaminate the connector.

Step 5: Inspect the Wiring Harness

Follow the harness from the sensor toward the ECM.

Look for:

A grounded signal circuit commonly causes P0117.

Step 6: Disconnect the Sensor and Watch Live Data

With the ignition on, disconnect the coolant temperature sensor.

On many vehicles, the scan-tool temperature should drop to an extremely cold reading, often around -40°F.

If disconnecting the sensor changes the reading from extremely hot to extremely cold, the sensor itself may be shorted internally.

If the reading remains extremely hot with the sensor disconnected, the signal wire may be shorted to ground.

Always follow the manufacturer’s diagnostic procedure.

Step 7: Test Sensor Resistance

Disconnect the ECT sensor and measure resistance using a multimeter.

Resistance should generally:

Compare the results with the manufacturer’s temperature-versus-resistance chart.

A sensor showing near-zero resistance when cold may be internally shorted.

Step 8: Check the Signal and Ground Circuits

Verify:

A damaged sensor ground usually causes high-voltage readings, while a signal wire shorted to ground commonly causes low-voltage readings.

Step 9: Diagnose the Cooling System

If the temperature reading is accurate and the engine is genuinely overheating, inspect:

P0117 may be reporting a real temperature problem rather than an electrical fault.

Step 10: Verify the Repair

After completing repairs:

  1. Refill and bleed the cooling system if opened.
  2. Reconnect all electrical components.
  3. Clear the code.
  4. Start the engine cold.
  5. Monitor coolant temperature.
  6. Confirm the reading rises gradually.
  7. Verify thermostat operation.
  8. Confirm cooling fan operation.
  9. Road test the vehicle.
  10. Rescan for stored or pending codes.

The temperature reading should rise smoothly without sudden jumps.

Scan-Tool Clues for P0117

Useful clues include:

Scan ReadingPossible Cause
Extremely high temperature before startupShorted sensor or wiring
Reading changes when connector is unpluggedFailed sensor likely
Reading stays extremely high unpluggedSignal wire shorted to ground
Actual engine is overheatingCooling-system fault
Reading jumps while moving harnessIntermittent wiring or connector issue
Cooling fans run constantlyECM fail-safe response

Common P0117 Diagnostic Mistakes

Assuming the Sensor Is Always Bad

The sensor is common, but shorted wiring or actual overheating can create the same code.

Ignoring the Cooling System

Always verify the engine is not truly overheating before focusing only on electrical diagnosis.

Replacing the Thermostat Without Testing

A thermostat may cause overheating, but it will not repair a shorted sensor circuit.

Confusing Circuit Voltage With Temperature

“Low input” means low electrical voltage, not low engine temperature.

Replacing the ECM Too Early

ECM failure is rare. Test the sensor, wiring, connector, and cooling system first.

Using Only the Dashboard Gauge

The gauge may use a different sensor or heavily filtered ECM data. Scan-tool readings provide more useful diagnostic information.

Vehicle-Specific P0117 Problems

Chevrolet and GMC

Common causes include:

Common models include:

Ford

Common problems include:

Common models include:

Some Ford vehicles use a Cylinder Head Temperature sensor rather than a coolant-mounted sensor.

Toyota and Lexus

Typical causes include:

Common models include:

Honda and Acura

Common problems include:

Common models include:

Nissan and Infiniti

Common causes include:

Common models include:

Hyundai and Kia

Typical causes include:

Subaru

Common problems include:

BMW

Common causes include:

Volkswagen and Audi

Typical issues include:

P0117 Repair Costs

RepairEstimated Cost
Connector cleaning$10–$75
Coolant temperature sensor$50–$250
Wiring repair$100–$400
Connector replacement$75–$300
Thermostat replacement$150–$500
Cooling fan repair$200–$900
Water pump replacement$400–$1,500
Head gasket repair$1,500–$4,000+
ECM repair or replacement$800–$2,000+

Related Trouble Codes

P0117 may appear with:

P0117 indicates a low-voltage signal, while P0118 indicates a high-voltage signal.

Frequently Asked Questions

Does P0117 mean the engine is overheating?

Not always.

It means the ECM is receiving a low-voltage signal that may indicate very high coolant temperature. The cause may be actual overheating, a failed sensor, or a shorted circuit.

Can I drive with P0117?

Only after confirming that the engine is not overheating.

If the temperature gauge is high, coolant is leaking, or steam is present, stop driving immediately.

Can a bad coolant temperature sensor cause P0117?

Yes.

An internally shorted coolant temperature sensor is one of the most common causes.

Can low coolant cause P0117?

Low coolant can contribute to actual overheating or unstable sensor readings, but P0117 is usually caused by an electrical low-voltage condition.

Can a bad thermostat cause P0117?

Yes, indirectly.

A thermostat stuck closed can cause genuine overheating, which may result in a low ECT sensor voltage. However, a thermostat does not typically cause an electrical circuit fault by itself.

Why are my cooling fans running constantly?

The ECM may activate the fans as a fail-safe if it believes the engine is overheating or cannot trust the coolant temperature signal.

Why does my scan tool show 300°F?

An extremely high reading on a cold engine usually indicates a shorted ECT sensor or signal wire.

Can P0117 cause hard starting?

Yes.

If the ECM believes the engine is already extremely hot during a cold start, it may reduce fuel enrichment too much, causing extended cranking or rough starting.

Will replacing the sensor fix P0117?

It will if the sensor is shorted internally.

It will not fix damaged wiring, connector corrosion, an actual overheating condition, or a short to ground elsewhere in the circuit.

Final Thoughts

The P0117 Engine Coolant Temperature Sensor Circuit Low Input code means the ECM is receiving a coolant temperature signal voltage that is lower than expected. This may indicate a shorted sensor, grounded signal wire, damaged connector, or an engine that is actually overheating.

The first step is always to verify the engine’s true temperature and check coolant level. If the engine is not overheating, compare live coolant temperature data with ambient temperature and inspect the sensor connector and wiring.

A scan-tool reading near 300°F on a cold engine strongly suggests an electrical fault. By diagnosing the sensor circuit before replacing parts, you can avoid unnecessary cooling-system repairs and determine whether P0117 is merely lying about the temperature—or warning you that the engine is genuinely trying to melt itself.

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