How to recognise air in the circuit or a stuck thermostat (P0128), how to bleed it properly and which signs point to a head gasket.
Air left in the cooling circuit or a stuck thermostat causes a temperature that rises and falls, a heater blowing cold or P0128. Learn how to check the thermostat with a scan tool, how to bleed the circuit properly and which signs point to a more serious problem.
The cooling circuit only works properly when it is full of the right coolant and free of air pockets. After replacing a pump, thermostat or hose, trapped air collects at the high points, blocks circulation and causes local overheating even when the expansion tank looks full. The thermostat itself can stick open (the engine does not warm up) or closed (the engine overheats).
Original editorial schematic, illustrative only — not a real scan-tool screenshot and not a substitute for engine-specific procedures.
Work only on a cold engine, use coolant to the manufacturer's specification and watch the real temperature in a scan tool. An engine that overheats repeatedly after bleeding has another problem, not air.
On a phone, scroll the table to see all columns.
2. Main causes| Cause | Typical symptom | Check |
|---|---|---|
| Air in the circuit after work | Fluctuating temperature, cold heater, gurgling | Correct bleeding, level dropping at first |
| Thermostat stuck open | Engine slow to warm up, P0128, higher consumption | Temperature in the scan tool on a long run |
| Thermostat stuck closed | Rapid overheating, hose to the radiator cold | Hose temperatures, scan-tool temperature |
| Faulty water pump or fan | Overheating at idle or in traffic | Does the fan start at its threshold? Leaks at the pump? |
| Head gasket | Continuous bubbles in the tank, high pressure, coolant loss with no visible leak | CO₂ test in the coolant, pressure test |
Start only with a cold engine, ideally after a few hours standing.
Warning: Never open the expansion tank cap when the engine is hot: pressurised coolant can spray out and cause serious burns.
The level should sit between MIN and MAX with a cold engine. Use only coolant to the specification in the manual; do not mix types and do not top up with tap water.
Start the engine and watch coolant temperature in live data. On the road, an engine with a good thermostat reaches operating temperature and holds it; if it stays well below, the thermostat is stuck open.
Fit the bleeding funnel to the expansion tank and open the bleed screws the manufacturer specifies (on hoses, radiator or thermostat housing). Some engines have a specific bleeding procedure with an electric pump activated from the scan tool.
Set the heater to maximum temperature, fill the funnel slowly and let the engine idle, then run at slightly raised revs. Close each bleed screw when coolant comes out without bubbles.
Watch the temperature until the thermostat opens: the hose to the radiator gets warm and the funnel level drops suddenly. Continue until no more bubbles rise and the radiator fan has run at least once.
Switch off, remove the funnel once cool and set the level to MAX. Check again the next day with a cold engine: a small drop after bleeding is normal.
If there were codes (for example P0128), clear them and take a confirmation drive while watching temperature in the scan tool. If the level keeps falling or bubbles never stop, check the head gasket.
Free: 5 reports a month
Save these diagnostic steps and create a professional technical report with your logo and workshop details to hand to your customer as a PDF: codes read, findings, photos and recommendations. Import the codes from a photo of the scan tool and issue the report in 13 languages, in the customer's currency.
Create the diagnostic PDF reportImport codes from a photoTick off completed checks. The list resets when you reload the page.
0 / 8 checks completed
Engine cold before opening the tank Coolant of the correct specification Temperature watched in the scan tool Bleed points opened and closed in turn Heater on full during bleeding Thermostat open and fan has run Level rechecked cold the next day Codes cleared and confirmation drive doneUsually until the thermostat opens and the fan starts, so tens of minutes. Some engines have longer special procedures.
Can I drive if the heater blows cold?A cold heater can be the first sign that coolant is not circulating. Watch engine temperature and do not continue if it rises above normal.
How do I know the thermostat is stuck?Stuck open: the engine warms very slowly and stays below normal on the road (P0128). Stuck closed: rapid overheating with the radiator hose cold.
Do you offer diagnostics, repairs or vehicle inspections?
Discuss a partnership with OBDAuto.roDiagnostics and OBD codes
Scanners, connection and correct code clearing; live data and freeze-frame explained. Compare ELM327, Bluetooth and professional tools, with official screenshots and criteria for real tests.
Diagnostics and OBD codes Cheap ELM327 scanner: how to use it, which apps are worth it and the risksChips, protocols and adapters compared honestly, apps matched to makes and the clear limits of universal OBD diagnostics.
Diagnostics and OBD codes Dacia Logan II fault codes: symptoms and checksOBD examples for Dacia Logan II, grouped by engine: misfires, mixture, turbo, EGR and DPF. Explanations and diagnostic checks.
Diagnostics and OBD codes Dirty or faulty air mass meter (MAF): symptoms, testing and proper cleaningWhich values to check in the scan tool, how to rule out air leaks, how to clean the sensor without destroying it and when to replace it.