The alternator warning lamp circuit has failed. The lamp itself is part of the excitation circuit on many cars, so a blown bulb can stop the alternator charging altogether.
P0621 — also written PO621, P 0621, 0621 — is the OBD-II code for generator lamp / l terminal circuit. Same code, same fix: this page covers every spelling of it.
P0621 is a electrical fault code, typically costing £80–£600 to put right in a UK garage.
What causes P0621?
- Blown dash warning bulb or failed LED driver
- Failed alternator regulator
- Broken wire to the alternator L terminal
How P0621 is fixed
- Check the warning lamp operates at ignition on
- Test charging voltage at the battery
- Replace the regulator or alternator
- Repair the L terminal wire
P0621 repair cost in the UK
Most P0621 repairs land between £80–£600 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Can you drive with P0621?
If the battery light is on as well, you are running on battery alone. Get home and stop.
Urgency: high.