Engine overheating
Temp gauge climbing into the red. What to monitor via OBD-II, which codes to watch.
Overheating is critical — don't keep driving (head-gasket / warped head territory). But with the right diagnostics you can pinpoint the cause in ~20 minutes.
Causes by frequency: (1) thermostat stuck closed, (2) cooling fan not engaging (relay/sensor), (3) low coolant from a leak (radiator / hose / head gasket), (4) failing water pump, (5) clogged radiator outside (bugs/debris).
What OBD-II shows: ECT live data (Mode 01 PID 05). Watch the graph while driving. Linear temperature rise without dip = fan or thermostat. Jumping = low coolant with an air pocket.
First diagnostic steps
- Stop immediately and let cool (minimum 30 min)
- Check coolant level in the overflow tank
- Read live ECT — if ECU shows 265°F but tank is only warm, sensor is bad or coolant is gone
- Inspect radiator externally (bugs, leaves)
- Run ARR test (Mode 06 on modern cars) for thermostat monitor
Likely fault codes
Click a code for detailed description, causes, and diagnostic plan.