Drivable — worth looking into
The heater blows cold
The engine is warm but the heater blows cool or barely warm air. Sometimes one side is warmer than the other; sometimes it only gets warm while driving.
There is no danger, but the symptom will not go away by itself: it is the early stage of something that gets pricier.
What to look at in the live data
- Coolant temperature with the engine warm. If it never reaches the working range and drops on the motorway, the trouble is the thermostat, not the heater.
- Whether there are climate flap codes: the flap does not reach its commanded position and the heater freezes in one setting.
- How the temperature behaves in traffic and on the motorway: the difference tells a thermostat from low coolant.
Common causes, cheapest first
- Low coolant — checked by looking into the tank and costs nothing.
- A thermostat stuck open: the engine never warms up, so the heater cannot heat.
- An air lock in the system after topping up or a repair.
- A jammed climate flap — it clicks behind the dash when the ignition goes on.
- A blocked heater matrix — warm in, cold out.
Codes that show up with this
P0128Coolant thermostat — temperature below regulating temperatureB1200Climate control module — air door malfunction
These parameters in the reference
Do it step by step
Weak heater and higher fuel use? One graph answers the question in ten minutes.How to tell whether the thermostat is to blame
A symptom is not a diagnosis. The same sign comes from a dozen different causes — the readings are what tell them apart, not the feeling.
How the app helps
CarScan shows exactly these readings live, remembers the highest and the lowest value of the trip, and puts them into a report you can hand to a mechanic.