Drivable — worth looking into
Slow to warm up, the heater blows cold
The temperature needle sits low for ages, the cabin stays cold, and on the motorway it gets worse. Consumption rises noticeably.
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 — the key parameter. Normal running is about 85–95 °C; below 70 after fifteen minutes of driving is a thermostat conversation.
- How the temperature behaves on the motorway: falling while moving is an almost certain sign of a thermostat stuck open.
- Fuel trims: on an engine that never warms up the unit keeps the mixture rich.
Common causes, cheapest first
- A thermostat stuck open — the most common cause.
- Low coolant or air in the system.
- A blocked heater matrix.
- A temperature sensor that lies (then the gauge and the data disagree).
Codes that show up with this
P0128Coolant thermostat — temperature below regulating temperatureP0117Engine coolant temperature circuit low inputP0118Engine coolant temperature circuit high input
These parameters in the reference
Coolant temperatureShort term fuel trim B1Long term fuel trim B1Engine oil temperatureAmbient air temperatureTime since engine startWarm-ups since codes cleared
Lamps that may be on
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.