GM Duramax · Engine Coolant Temperature CircuitMedium

GM Duramax P0117: Diagnosing a Low Coolant Temperature Sensor Reading

P-Code P0117 (FMI N/A) — Engine Coolant Temperature Circuit Low Input

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Quick Answer

P0117 on GM Duramax engines means the coolant temperature sensor circuit is reading below normal, consistent with a short to ground. It's a medium-severity fault that mainly affects cooling fan control and cold-start fueling accuracy.

Overview: GM Duramax P-Code P0117

Coolant temperature is one of the most heavily-used sensor inputs on the whole engine, so when P0117 shows this circuit shorted low, it affects more than just the dash gauge. ProfessionalDieselRepair.com treats this fault as worth fixing promptly - not because it's dangerous, but because an inaccurate cold reading causes the cooling fan to run more than necessary and can throw off cold-start fueling precision.

Repair Snapshot

Time

0.5 - 1.5 Hours

Difficulty

Beginner

Parts Cost

$20 - $90

Dealer Labor

$150+

Tools Required

  • Diesel Laptops Scan Tool
  • Multimeter
  • Jaltest Diagnostic Software
  • Wire Repair Kit

What P-Code P0117 Means

P0117 means the coolant temperature sensor circuit is reading voltage below normal, consistent with a short to ground. On a Duramax this is a medium-severity fault - the PCM will default to a conservative fueling and cooling fan strategy since it can't trust a reading pegged at an unrealistic cold value.

Symptoms Drivers Notice

  • Cooling fan running excessively, even when the engine isn't actually hot
  • Check Engine Light illuminated
  • Slightly harder cold starts if the default fueling strategy isn't optimal
  • Cluster temperature gauge reading inaccurately low

Most Likely Root Causes

Coolant-contaminated or corroded sensor connector

Common

A weeping intake manifold gasket or nearby coolant leak can contaminate the connector, creating a low-resistance path that mimics a sensor fault.

Chafed wiring shorting the signal to ground

Occasional

Harness wear near the engine can create a short-to-ground path on the signal circuit independent of the sensor itself.

Failed sensor element

Occasional

Internal sensor failure can independently produce a shorted-low reading, though it's less common than connector contamination in this location.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor without checking for a coolant leak at the connector

Why it happens: It's a quick, inexpensive swap that seems like the obvious fix - a small parts-cannon habit that skips an easy inspection step.

Do this instead: Check for coolant contamination at the connector first, since a nearby leak causing corrosion is a common and easily fixable root cause.

Master Tech Commentary

Mechanic's Notes

Shop tip: check for coolant seepage at the connector, which is a common contributor to corrosion on this specific sensor. ProfessionalDieselRepair.com always inspects for a weeping intake manifold gasket or nearby coolant leak before assuming the sensor itself has failed.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the code with Diesel Laptops and confirm P0117 is active; check the reported coolant temperature, which will likely be pegged at an extreme cold value.
  2. Step 2: Locate the coolant temperature sensor and inspect the connector for corrosion, coolant residue, or a loose pin.
  3. Step 3: Disconnect the sensor and check the signal wire for continuity to ground with the harness isolated from the PCM.
  4. Step 4: Test the sensor's resistance at a known temperature and compare against the manufacturer's resistance-versus-temperature chart.
  5. Step 5: Repair any chafed or corroded wiring found, or replace the sensor if it fails the resistance test.
  6. Step 6: Clear the code and confirm the reported coolant temperature climbs realistically during a warm-up cycle before returning the truck to service.

Live Data & Freeze-Frame Tips

Watch the reported coolant temperature on the scan tool through a full warm-up cycle - a value pegged at an extreme cold reading regardless of actual engine warm-up confirms the circuit fault rather than a legitimate reading.

Safety Warnings

  • Allow the engine to cool completely before accessing the coolant temperature sensor.
  • Never remove a coolant system cap while the engine is hot.
  • Dispose of any drained coolant per local environmental regulations.

When to Limp In vs Tow

Towing isn't necessary for this fault - it doesn't directly affect drivability, though it should be repaired promptly to restore accurate cooling fan and fueling behavior.

Prevention Tips for Fleets & Owner-Operators

  • Inspect the coolant temperature sensor connector for leaks or contamination during routine service.
  • Address any coolant leaks near the sensor promptly to prevent connector corrosion.
  • Apply dielectric grease to the connector when reassembling after any inspection.

Related Fault Codes

P0118

Engine coolant temperature circuit high input - the opposite electrical failure mode, typically a short to power or an open circuit.

P0116

Engine coolant temperature circuit range/performance - a plausibility fault distinct from this specific low-voltage electrical fault.

P0128

Coolant thermostat below regulating temperature - a related but mechanically distinct fault that can sometimes accompany sensor issues.

Frequently Asked Questions

Can I drive with P0117 active?

Yes, this fault doesn't directly affect drivability, though the cooling fan may run more than necessary. Repair it to restore accurate monitoring and normal fan behavior.

How do I reset P0117 without a scan tool?

You need Diesel Laptops or a comparable scan tool to clear the stored fault after the sensor circuit is repaired.

Will this code cause an engine derate?

No, coolant temperature sensor circuit faults don't trigger a power derate - they primarily affect fan control and fueling strategy accuracy.

Could a coolant leak really cause this electrical fault?

Yes, coolant seepage at the connector is a surprisingly common cause of corrosion that produces exactly this type of shorted-low reading.

Is the coolant temperature sensor expensive to replace?

No, it's a relatively inexpensive sensor, but check for connector contamination first since that's a common and easily fixable cause.

Will excessive fan running damage anything?

It can accelerate wear on the fan clutch and belt over time if left unaddressed for an extended period, so it's worth fixing rather than ignoring.