P0118 on Chevrolet: How to Diagnose & Fix
Engine Coolant Temperature Sensor Circuit High
1 Chevrolet model · 1 documented issue · $150-$600 repair
P0118 on Chevrolet by model: symptoms, diagnosis and fix
On the 2.2L/2.4L Ecotec the thermostat is integrated into a plastic housing on the back side of the engine facing the firewall, and the water pump sits behind the timing cover on the passenger side. Both are recurring leak points. The rear thermostat housing seal hardens and the housing itself corrodes and seeps, and because the leak is tucked against the firewall the coolant evaporates on the hot block instead of dripping — so owners chase a steadily dropping reservoir with no puddle in the driveway, a complaint that appears over and over on the Cobalt forums. The water pump leaks at its gasket toward the passenger side, and because it is timing-chain driven, a leaking pump can wash the front of the engine and contaminate the chain area. A slow leak that runs the car low leads to overheating and, on the Ecotec, a warped or cracked head.
Common Symptoms
- Coolant reservoir slowly drops with no visible puddle
- Sweet coolant smell after driving
- White crust or dried residue at the back of the engine near the firewall
- Temperature gauge climbing or heater blowing cold
- Overheat warning in traffic
How to Fix
Pressure-test the cooling system cold at about 15 psi and inspect the back of the engine with a light and mirror while it holds pressure — that is where the thermostat housing lives and where the leak hides. Replace the thermostat housing as an assembly with a new seal rather than reusing a corroded housing. If the leak is on the passenger side near the timing cover, the water pump and its gasket are the likely source; replace the pump with an OE-quality unit and refill with the correct Dex-Cool. After repair, burp the system properly — the Ecotec traps air and an unbled system overheats and mimics the original fault. If you find oil in the coolant reservoir, stop and test for head gasket failure before spending money on the pump.
Sources for this issue (3)
P0118 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0118 mean on a Chevrolet?
P0118 stands for "Engine Coolant Temperature Sensor Circuit High." P0118 indicates the engine coolant temperature (ECT) sensor circuit is reading abnormally high voltage, which the computer interprets as an implausibly cold coolant temperature. The ECT sensor changes resistance with temperature and tells the PCM how hot the engine is so it can manage fuel, the cooling fans, and the gauge. High voltage on this circuit usually points to an open circuit or a sensor reporting an unrealistically low temperature. The car may run rich, struggle to enter closed-loop operation, idle poorly, or show an inaccurate temperature gauge. On Chevrolet it is documented across 1 model: Cobalt.
What are the most common causes of P0118?
The most common causes of P0118 are: Faulty (open) engine coolant temperature sensor, Open or broken wire in the ECT signal circuit, Loose, corroded, or disconnected sensor connector, Poor ground or open in the sensor return circuit, Damaged wiring harness near the sensor, Corrosion in the connector terminals, Failed PCM input (rare). The specific cause varies by vehicle.
How much does it cost to fix P0118 on a Chevrolet?
Published repair estimates on Chevrolet range from $150 to $600 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0118 on a different make?
View P0118 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.