P0621
Generator Lamp/L Terminal Circuit
- Severity
- Medium
- Driving
- Drivable — book it in
A fault in the generator lamp or L-terminal circuit. The L terminal is the connection that lights the battery warning lamp and, on many alternators, also provides the initial excitation the alternator needs before it will start charging. That second job is why this code sometimes means no charging at all rather than just a lamp fault.
Possible causes
- CommonBlown battery warning lamp bulb - on older systems this alone stops the alternator exciting
- CommonWiring or connector fault on the L terminal circuit
- CommonFailed alternator regulator
- PossibleCorroded connector at the alternator
- PossibleFailed instrument cluster lamp driver
What you might notice
- Battery warning light not working, or permanently on
- No charging at all
- Battery going flat
- Check engine light on
What to check, in order
- Check whether the battery warning lamp lights during the ignition-on bulb check. If it never lights and the vehicle is not charging, the bulb may be the whole fault - a genuinely cheap repair that is easy to overlook.
- Measure charging voltage at the battery to establish whether the alternator is working at all.
- Inspect the connector at the alternator for corrosion and heat damage.
- Check continuity on the L terminal wire between the alternator and the cluster.
- Check the earth connections at the alternator.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Computer output circuits and control module
P0600P0601P0602P0603P0604P0605P0606P0607P0608P0609P0610P0611P0612P0613P0614P0615P0616P0617P0618P0619P0620P0621P0622P0623P0624P0625P0626P0627P0628P0629P0630P0631P0632P0633P0634P0635P0636P0637P0638P0639P0640P0641P0642P0643P0644P0645P0646P0647P0648P0649P0650P0651P0652P0653P0654P0655P0656P0657P0658P0659P0660P0661P0662P0663P0664P0665P0666P0667P0668P0669P0670P0671P0672P0673P0674P0675P0676P0677P0678P0679P0680P0681P0682P0683P0684P0685P0686P0687P0688P0689P0690P0691P0692P0693P0694P0695P0696P0697P0698P0699