While driving this afternoon, my temperature gauge shot immediately to full hot at the top of the red range. I pulled over as soon as I could and checked for leaks, and there were none. The engine didn’t seem to be any hotter than normal. I assumed it might be an incorrect gauge and drove straight home. After being parked for about two hours I checked the temperature again, and it was still hot. What do you think is wrong?
You did a great job explaining how this all unfolded. It sounds like the temperature gauge circuit has encountered a fault that caused it to read at its limit, regardless of actual engine temperature. You were wise to take the reading at face value initially, because an overheating engine can lead to incredibly expensive repair costs. Because the gauge needle moved immediately to hot and continued to stay there with a cooled-down engine, we can suspect it’s likely the only problem — but it’s best to play it safe.
Verifying the operating temperature of an engine can be done in several ways. The most accurate is to connect an inexpensive OBD-II generic scan tool, which costs $50-$100 at most auto parts stores and online, to the vehicle’s data link connector, typically found under the instrument panel on the driver’s side. The scan tool’s data list will include engine temperature, obtained by the engine management system’s highly accurate temperature sensor. Some vehicles employ separate engine coolant temperature sensors for the management system and instrument panel gauge, or the information from a single sensor may be shared.
A note of caution: Some vehicles will display a seemingly correct, default (made-up) scan tool data value when the actual sensor reading is deemed invalid. It’s meant to provide proper engine performance but can be misleading.