Engine Overheating: Causes, Likely Codes & What to Do
TL;DR
Engine overheating = coolant temperature too high. Severity: CRITICAL — stop driving to avoid engine damage. Related codes: P0128 (thermostat), P0118/P0117 (coolant temp sensor). Top causes: low coolant/leak, stuck thermostat, failed water pump, bad cooling fan. Let it cool, then diagnose.
What "engine overheating" means
The cooling system circulates coolant through the engine and radiator to carry heat away. Overheating happens when that flow or heat transfer fails: not enough coolant, a thermostat stuck closed, a pump not circulating, or a fan not pulling air at low speed. Sustained overheating warps cylinder heads and blows head gaskets, so it is one of the few problems where you should pull over immediately.
Quick diagnosis: IF → THEN
| If… | Then… |
|---|---|
| the temperature gauge is in the red or you see steam | pull over safely, shut the engine off, and let it cool — do not keep driving |
| it overheats in traffic but is fine at highway speed | suspect the radiator cooling fan not running at low speed |
| it overheats at highway speed or all the time | suspect low coolant, a stuck thermostat, or a failing water pump |
| coolant is disappearing with no visible leak | suspect an internal leak (head gasket) — have it checked promptly |
Most likely fault codes
| Code | Likelihood | Notes |
|---|---|---|
| P0128 — Coolant thermostat (below temp) | 35% | Thermostat faults relate to temperature control |
| P0118 — Coolant temp sensor circuit high | 30% | |
| P0117 — Coolant temp sensor circuit low | 20% | |
| P0300 — Misfire (can follow head-gasket damage) | 15% |
Common causes
- Low coolant level or a coolant leak
- Thermostat stuck closed
- Failing water pump (not circulating coolant)
- Radiator cooling fan not running
- Clogged radiator or, in serious cases, a head-gasket failure
What to do
- Stop driving and let the engine cool before opening anything — hot coolant is dangerous.
- Once cool, check the coolant level and look for leaks (hoses, radiator, water pump).
- Verify the radiator fan turns on when the engine is hot.
- Have the thermostat and water pump tested if coolant and fan are okay.
- If coolant vanishes with no leak, have a head-gasket / combustion test done.
When is it urgent?
Overheating is a CRITICAL, drive-stopping problem. Continuing to drive a hot engine can warp the head or blow the head gasket within minutes, turning a cheap fix into a major repair. Pull over and let it cool.
Frequently asked questions
Can I keep driving if my engine is overheating?
No. Continuing to drive an overheating engine can permanently damage it — warped heads or a blown head gasket — within minutes. Pull over safely and let it cool.
What is the most common cause of overheating?
Low coolant (often from a leak), a stuck thermostat, a failing water pump, or a cooling fan that is not running. These cover the large majority of cases.
Why does my car overheat in traffic but not on the highway?
At low speed the radiator relies on the cooling fan for airflow. If the fan is not running, the engine overheats in traffic but cools at highway speed when air flows through the radiator naturally.