OBD-II Diagnostic Trouble Code
P0118 — Engine Coolant Temperature Circuit High
P0118 means the ECT signal voltage is too high, which the ECM reads as an extremely cold engine. This usually comes from an open circuit: an unplugged sensor, a broken wire or ground, or an open sensor.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Engine Coolant Temperature Circuit High
Code
P0118
Category
Powertrain
Subsystem
Cooling
Truck-relevant
Yes
What this means for truckers
P0118 (Engine Coolant Temperature Circuit High) — opposite of P0117. ECT sensor reading impossibly hot. Same root causes (sensor or wiring), but on this one the ECM goes into protect mode and may richen the mixture or reduce power. If the temperature gauge says 'cool' but P0118 is set, the sensor is bad — don't trust the gauge alone.
Before you replace parts — try this first
P0118 on the Cooling system is most often unplugged or loose ect connector — 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 cold or at the bottom, fan may run all the time as a fail-safe, and you may see extra white smoke, rough idle, longer intake heater cycles, poor fuel economy, and regen inhibit on diesels.
Common causes
- CommonUnplugged or loose ECT connector ($20–$150)
- CommonOpen signal or ground wire ($80–$400)
- CommonOpen (failed) ECT sensor ($80–$300)
- OccasionalCorroded connector pins ($30–$200)
How to fix
Check that the connector is fully seated and locked
When to use: First step, especially after cooling system work
Jumper the signal and ground at the harness; a very hot reading means the wiring is good
When to use: To separate an open sensor from an open wire
Check signal and ground continuity to the ECM
When to use: When the jumper test does not change the reading
Replace the ECT sensor
When to use: When the sensor reads open
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 P0118?
P0118 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 P0118 on a commercial truck?
Total repair cost ranges $20–$400 depending on the root cause. The cheapest fix is usually the first one listed below — Unplugged or loose ECT connector — 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 P0118 fail a DOT inspection?
P0118 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 P0118 and related codes?
P0118 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 P0118 is set without the companions, the issue is more isolated.
How do I prevent P0118 from coming back after a repair?
For P0118 (Engine Coolant Temperature Circuit High), 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.