You can drive, but do not put it off
The voltage jumps, the lights dim
The headlights and the dashboard change brightness while driving, sometimes the battery lamp flickers. The electronics behave oddly.
You can usually get home or to a garage, but every day of delay adds to the bill.
What to look at in the live data
- System voltage with the engine running: usually 13.8–14.6 V and steady.
- How it behaves under load: switch on the lights, the heated glass and the blower — below 13 V the alternator is short of capacity.
- Alternator circuit and system voltage codes.
- Voltage while cranking: below 9–10 V is about the battery, not the alternator.
Common causes, cheapest first
- Corroded terminals and a bad earth — free, and checked first.
- Worn brushes or the alternator's voltage regulator.
- A stretched or oily belt: the alternator slips under load.
- A dying battery: it stops smoothing the peaks.
Codes that show up with this
These parameters in the reference
Lamps that may be on
Do it step by step
Three voltage readings answer the question of who is to blame: the battery, the alternator or the wiring.How to check the battery and alternator with the app
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.