Fault Tree: Overheating

Quick triage & tools

Scan first: read codes (all modules if you can) + live data that matches the symptom.
Power basics: battery voltage (resting + while cranking) and quick fuse checks before deep dives.
Have ready: basic scan tool, multimeter, and a way to confirm the symptom (road test / idle test).

Tip: if a step says “check wiring”, use Wiring diagrams basics → and Voltage drop testing →

Stop rules (don’t cook the engine)

If the temperature is climbing quickly, heater goes cold, or you see steam: stop and let it cool.
Never open the cap hot. Pressure can cause boiling/eruption.

Decision flow

Coolant level & obvious leaks
Check cold. Inspect undertray, radiator end tanks, hoses, thermostat housing, water pump area, heater core smell/fogging.

If low: top up with correct mix, but treat as a leak investigation. Use smoke testing principles (or a cooling system pressure tester if you have one).
Is the temperature reading believable?
- Scan live data coolant temp. Compare to dash gauge behaviour.
- Cold start: coolant temp should rise smoothly. Sudden jumps → sensor/wiring/ground.
Use: 5V reference & sensor grounds →
Fans: do they come on when they should?
- No fan when hot / AC on → check fan fuse/relay, resistor/module, fan motor, command from ECU.
- Fan runs but still overheats at speed → suspect flow/pressure, not airflow.
Thermostat stuck?
- Upper hose heats quickly but lower stays cold for too long → thermostat may not open.
- Heater output helps judge flow (but blend doors can mislead).
Use: Coolant temp & thermostat checks →
Radiator flow restriction
Cold spots across the radiator core (after warm-up) can indicate blockage. External fins packed with debris can also reduce heat rejection.
Pressure control / air in system
- Cap not holding pressure → boils early and overheats.
- Air pocket after coolant work → erratic gauge, heater goes cold then hot.
Head gasket indicators (don’t guess — verify)
- Pressurises rapidly from cold, repeated coolant loss, bubbles in expansion tank, exhaust smell in coolant.
- Misfire on startup with coolant loss, milky oil (not always present).
If you see these: do a block test / pressure test / compression or leak-down as appropriate.

Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.