OBD-II Diagnostic Trouble Code
P0117 — Engine Coolant Temperature Circuit Low
P0117 means the ECT signal voltage is too low. A low thermistor voltage reads as a very hot engine, so this usually means a signal wire shorted to ground or a shorted sensor, not a real overheat.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Engine Coolant Temperature Circuit Low
Code
P0117
Category
Powertrain
Subsystem
Cooling
Truck-relevant
Yes
What this means for truckers
P0117 (Engine Coolant Temperature Circuit Low) — see above.
Before you replace parts — try this first
P0117 on the Cooling system is most often ect signal wire shorted to ground — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Gauge may read very hot, fan runs all the time, and the ECM may trigger engine protection with a derate or shutdown warning. Hard cold starts are possible because the ECM thinks the engine is hot.
Common causes
- CommonECT signal wire shorted to ground ($80–$400)
- CommonInternally shorted ECT sensor ($80–$300)
- OccasionalCoolant or water in the connector ($30–$200)
- RareECM input fault ($900–$2500)
How to fix
Check actual coolant temperature with an infrared thermometer or test gauge
When to use: First step to rule out a real overheat
Unplug the sensor; if the reading drops to very cold, the sensor is shorted
When to use: To separate sensor from wiring
Check the signal wire for a short to ground
When to use: When the reading stays hot with the sensor unplugged
Replace the ECT sensor
When to use: When sensor resistance is far below spec
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0100 — Mass or Volume Air Flow Circuit
- → P0101 — Mass or Volume Air Flow Circuit Range/Performance
- → P0102 — Mass or Volume Air Flow Circuit Low Input
- → P0103 — Mass or Volume Air Flow Circuit High Input
- → P0104 — Mass or Volume Air Flow Circuit Intermittent
Frequently Asked Questions
Can I drive with code P0117?
P0117 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0117 on a commercial truck?
Total repair cost ranges $30–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — ECT signal wire shorted to ground — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0117 fail a DOT inspection?
P0117 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0117 and related codes?
P0117 is closely related to: P0100 (Mass or Volume Air Flow Circuit), P0101 (Mass or Volume Air Flow Circuit Range/Performance), P0102 (Mass or Volume Air Flow Circuit Low Input), P0103 (Mass or Volume Air Flow Circuit High Input). These codes share the same subsystem (cooling) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0117 is set without the companions, the issue is more isolated.
How do I prevent P0117 from coming back after a repair?
For P0117 (Engine Coolant Temperature Circuit Low), prevention centers on the cooling system. Service cooling system at OEM interval. Use the correct coolant type — modern HD engines need ELC (Extended Life Coolant), not green/conventional. Pressure-test cap and system annually.