P0621Generator Lamp/L Terminal Circuit
- Generic OBD-II code · Powertrain
- How the car detects it
- The alternator regulator is linked to the engine module by an L (turn-on) line, an F (field) line reporting field load as a duty cycle, or on newer cars a LIN bus. P0621 concerns the L line: the engine module applies voltage through a high-side driver and so switches the regulator's field control on and off. GM's 2023 and 2025 engine modules check the output impedance – open at 200 kΩ or more, short to ground or 5 V at 0.5 Ω or less – with ignition on or engine running and at least 11 V, after 5 s (short to ground 15 s), without the check engine light as an 'emission neutral' diagnostic. The Saab 9-4X switches on the battery lamp 15 s after setting the code; Saab's T8 flags an open D+ line after 0.5 s and raises the idle speed. Low and high on this line are P2500 and P2501.
- What you may notice
- Battery warning lamp on
- Battery not or incorrectly charged, starting problems
- Raised or fluctuating idle
- Starting problems after a longer drive
- Likely causes
- LikelyL line or alternator connector open or shorted
- PossibleRegulator or alternator faulty
- PossibleGround problems between engine and body
- RareDriver in the module faulty
- Repair
- Medium · Usually a job for a workshop.Typical cost: €30 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.