OBD-II Diagnostic Trouble Code
P0115 — Engine Coolant Temperature Circuit
P0115 is a general circuit fault for the engine coolant temperature (ECT) sensor. The ECM saw an ECT signal it could not use, such as an open or short, without a specific high or low reading. The ECM relies on ECT for fueling, fan control, grid heater use, regen permission and engine protection.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Engine Coolant Temperature Circuit
Code
P0115
Category
Powertrain
Subsystem
Cooling
Truck-relevant
Yes
What this means for truckers
P0115 (Engine Coolant Temp Sensor Circuit) — ECT sensor circuit fault. ECM can't read coolant temp. Sensor: $25-100, easy fix.
Before you replace parts — try this first
P0115 on the Cooling system is most often corroded 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
Check engine light, fan running all the time, hard cold starts, white smoke on diesels, wrong or missing gauge reading, and possible derate or regen inhibit because the ECM uses a default temperature.
Common causes
- CommonCorroded or loose ECT connector ($30–$200)
- CommonDamaged ECT wiring ($80–$400)
- OccasionalFailed ECT sensor ($80–$300)
- RareECM input fault ($900–$2500)
How to fix
Inspect the ECT connector and harness for corrosion, coolant contamination and damage
When to use: First step on P0115
Compare ECT to ambient and other temperature sensors after an overnight cold soak
When to use: To check sensor accuracy
Measure sensor resistance against the temperature chart and check wiring continuity
When to use: To decide between replacing the sensor and repairing wiring
Replace the ECT sensor and bleed air from the cooling system
When to use: When the sensor is out of 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 P0115?
P0115 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 P0115 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 — Corroded 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 P0115 fail a DOT inspection?
P0115 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 P0115 and related codes?
P0115 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 P0115 is set without the companions, the issue is more isolated.
How do I prevent P0115 from coming back after a repair?
For P0115 (Engine Coolant Temperature Circuit), 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.