P0621Powertrain
Generator lamp — circuit malfunction
Where to start
The alternator lamp circuit. Often it really is the circuit and not the alternator: the lamp takes part in exciting the field, and a blown bulb on the dash can leave the alternator not charging at all. A penny part with expensive consequences.
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
We have a detailed piece on thisCharging and voltage: P0560, P0561, P0620, P0621, P0622
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineControl module and its circuits
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Control module and its circuits»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.