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DTC codes / GM codes / P1117

P1117 · GMEngine Coolant Temperature Below Minimum Voltage

P1117 on a GM vehicle means the engine control module (ECM) has detected an abnormally low voltage signal from the engine coolant temperature (ECT) sensor circuit, which it interprets as a temperature reading that is below the minimum plausible value. In plain terms, the computer is seeing a signal that looks like the coolant is far colder than reality, so it cannot trust the sensor for fuel and fan control.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering

Symptoms you may notice

  • The check engine light is illuminated, and often there are no other noticeable symptoms besides the warning light.
  • The temperature gauge may read cold or stay near the bottom even after the engine has warmed up.
  • The cooling fans may run constantly or not at all because the ECM cannot trust the coolant temperature.
  • Fuel economy may drop and the engine may run rich because the ECM thinks the engine is cold.
  • Cold-start drivability may be poor, with rough idle or hesitation until the engine warms.
  • In some cases the engine may be hard to start or may stall when coming to a stop.

Common causes, most likely first

  1. A faulty engine coolant temperature (ECT) sensor that has drifted or failed internally.
  2. A short to ground or chafed wire in the ECT sensor signal circuit, often near the sensor connector or along the harness.
  3. Corroded, loose, or water-intruded terminals at the ECT sensor connector.
  4. A poor ground connection at the sensor or at the ECM ground point, causing a low reference voltage.
  5. A defective ECM driver or internal circuit for the ECT sensor, though this is less common.
  6. Aftermarket or incorrect ECT sensor installed that does not match the GM calibration.
  7. Coolant contamination or low coolant level that prevents the sensor tip from contacting coolant, causing an erratic low reading.

How to diagnose and fix it

  1. Connect a scan tool and read the ECT sensor voltage and temperature values cold and after warm-up, comparing them to actual coolant temperature with an infrared thermometer.
  2. Inspect the ECT sensor connector and harness for corrosion, pushed pins, chafing, or signs of coolant intrusion, and repair as needed.
  3. Measure resistance across the ECT sensor terminals and compare to the GM temperature-resistance chart for the vehicle; replace the sensor if out of specification.
  4. Check the sensor signal circuit for a short to ground with a multimeter while wiggling the harness, and repair any damaged wire.
  5. Verify the sensor ground circuit has less than 0.1 volt drop to the ECM ground; clean or repair grounds if excessive.
  6. Clear the code and perform a road test with the scan tool recording ECT data to confirm the signal stays within normal range.
  7. If all wiring and the sensor test good, follow the GM diagnostic flow chart for P1117 to check the ECM and replace only if confirmed faulty.

Why a cold reading is not always the sensor

  • A short to ground in the ECT signal wire can mimic a failed sensor, so technicians who replace the sensor without checking the harness often see the code return.
  • Coolant level or air pockets in the cooling system can cause the sensor tip to lose contact with coolant and report a low temperature, especially after cooling system service.
  • The ECM may use the ECT signal to enable cooling fans and adjust fuel trim, so a low reading can cause fans to run constantly and fuel economy to suffer even when the gauge looks normal.
  • Aftermarket sensors with incorrect resistance curves are a common cause of repeat P1117 codes on GM vehicles, and installing an OEM-equivalent sensor is often the permanent fix.
  • A poor ground at the ECM can pull the entire sensor reference circuit low, so checking ground integrity is part of a complete diagnosis before condemning the ECM.

Can I keep driving with P1117?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy load or long trips because the ECM may run the engine rich and cooling fan operation may be incorrect. If the temperature gauge is erratic or the engine overheats, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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