P0621

Generator Lamp/L Terminal Control Circuit

Car Fault Codes Engine computer & electrical

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 2.5 hours

Typical cost SAR 100 to 2,000

What it means

The L terminal on the alternator is tied to the charge warning lamp circuit, and on many vehicles it is used to switch on the voltage regulator at start-up. The engine computer (ECU) found this circuit in an abnormal state, so the charge lamp may not work correctly or the alternator may not start charging.

Symptoms4

Charge light stays on or never lights up with the key on.
Check engine light on.
Weak battery and hard starting if charging stops.
Dim or flickering lights.

Possible causes5

Corroded or loose alternator connector at the L terminal.
Broken L terminal wire, or one shorted to ground or to voltage.
Faulty voltage regulator inside the alternator.
Fault in the charge lamp circuit in the instrument cluster on some vehicles.
Faulty control circuit inside the engine computer (rare).

How to diagnose5

Switch the key on without starting and confirm the charge lamp lights, then goes out after starting.
Measure battery voltage while running and compare with the manufacturer specification.
Inspect the alternator connector and L terminal for corrosion or broken pins.
Check continuity of the L terminal wire and its isolation from ground and voltage sources.
Have the alternator and regulator tested on a charging system tester.

How to fix5

Clean and secure the alternator connector.
Repair the broken or shorted L terminal wire.
Replace the alternator or regulator if proven faulty.
Repair the charge lamp circuit in the cluster if the fault is there.
Clear the codes and confirm the charge lamp works, charging voltage is stable and the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for a connector or wire repair, and reaches the upper end if the alternator must be replaced.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

The charge light does not come on with the key on. Is that normal?

No. On most vehicles it should light with the key on and go out after starting; if it stays dark, the L terminal circuit may be faulty.

Will the battery go flat with this code?

It can, if the fault stops the alternator from starting to charge, so watch battery voltage and repair the circuit soon.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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