P0118 OBD2 Code in 2009 Chevrolet Malibu: Diagnosis and Fix Guide

When you connected the sensor directly to the ECM, did it work? A reading of 0.6 volts is abnormal. You must test the ECT sensor with the engine cold—otherwise, ambient temperature and thermostat housing readings are unreliable. If your scan tool shows a coolant temperature of 176°F, what was the actual coolant temperature? Check the ECT connector for loose connections or corrosion. Use a scan tool like KOEO to wiggle the connector—this can reveal intermittent faults. Without cold engine testing or infrared verification, you cannot confirm if the sensor is working properly. If the gauge doesn’t update until running, that’s normal—but the P0118 code suggests an issue with signal voltage. A bad ECT sensor is a likely culprit.

Ambient temperature: 70°F, coolant temp: 97°F, IAT: 88°F. I used an infrared gun on the thermostat housing and got 95°F—consistent with readings. The wiggle test showed no response. When unplugged, the gauge read zero. This suggests a possible sensor failure or poor signal transmission.

The engine was running at 95°F and coolant temperature was 97°F—very close. These values are reasonable. However, without a factory-level or high-end aftermarket OBD2 scan tool with bidirectional controls and full PID access, it's difficult to fully diagnose the issue. The problem could be a communication fault between modules or a faulty gauge cluster. It’s recommended to take the vehicle to an ASE-certified diagnostic shop. A P0118 code may indicate intermittent sensor issues. With a bi-directional scan tool, you can command the temperature gauge to activate and observe its response. Without full system diagnostics, it's hard to pinpoint the root cause. Global OBD2 systems have limited data access.

Thanks for the help! I spent all night dismantling parts—no loose wires found. I disconnected the battery overnight and let the car sit for about 18 hours. This morning, I checked temperatures: ambient was 68°F, IAT at 70°F, coolant temperature reading 97°F. Infrared gun on thermostat housing showed 71°F. The readings are consistent—so it seems like there might be two faulty sensors? Possibly ECT and IAT?

IAT appears to be functioning correctly. For accurate testing, you don’t need the engine running—you can test ECT and IAT with the engine cold by comparing sensor readings to ambient temperature. When the engine is cold, backprobing the signal wire should show between 2.5V and 4.5V depending on ambient conditions. If you're getting near-zero voltage or a reading that jumps to ~5V when unplugged, this indicates a shorted ECT sensor. A P0118 code means the PCM detected high voltage on the ECT signal wire—this could be due to a faulty sensor or an open ground. Have you cleared the code and monitored if it returns? After clearing, jump both terminals of the ECT with a jumper—the scan tool should read 260–400°F. Then start the engine and observe gauge response.

I flushed the radiator and added new Dexcool coolant. It worked fine for about 50 miles—then the issue returned on my way to work today. I’m giving up and will take it to the dealer Monday. This car is becoming a 'project' vehicle.

Without proper diagnostic tools and a solid understanding of circuit design, accurate diagnosis online is extremely difficult. A qualified technician should be able to identify the issue quickly. Best of luck with your troubleshooting!

Thank you—will update everyone on what the dealer finds.

After a final effort, I took it to the dealer and bought a new thermostat. It's been in for four days now with no issues. All I did was drain the coolant and install the new thermostat—no battery disconnect or other work. As soon as I started the car, everything worked normally. How strange is that?!

That’s great to hear! A P0128 code would point to a thermostat issue. A P0118 code—high voltage on ECT—would typically indicate either a faulty sensor or an open ground in the wiring. You replaced the thermostat once, and it seems you may have resolved a hidden wiring or connection problem. A stuck-open thermostat simply takes longer to heat up. The ECT sensor provides the PCM with coolant temperature data for the gauge display.