Charging and voltage: P0560, P0561, P0620, P0621, P0622

The car starts every other time, the battery lamp flickers, and the list shows voltage and alternator codes. Here is how to tell a dead battery from an alternator that stopped receiving commands — in a minute.

The answer is one number. Start the engine, open control module voltage and look: 13.8–14.6 V means charging is happening. Below 13 and the alternator is not charging — the question is then why.

What the codes say

P0560 — system voltage malfunction, P0561 — unstable. The unit is complaining about its own supply. P0561 is the worse of the two: «jumping» is more dangerous than «low», because the swings drive every other module mad — and U codes, lost communication, appear alongside.

P0620 — alternator control circuit. Important: on a modern car the alternator does not live on its own. The unit tells it how much to produce, based on load, temperature and battery state. Break that command and the alternator either stops charging or charges flat out forever.

P0622 — field winding circuit, P0625 and P0626 — low and high signal. The field winding is how an alternator «switches itself on». Without it it spins for nothing.

P0621 — alternator lamp circuit. The most deceptive one: on many cars the dashboard lamp is not merely an indicator but part of the excitation circuit. A blown bulb and the alternator may never start charging at all. A penny part that gets cars towed.

The one-minute check

All of it is done in the app with the engine running, using control module voltage.

1. Right after starting: 13.8–14.6 V. If it reads 12.3–12.6, the alternator is not charging at all, the car is running on the battery and will soon stop.
2. Switch on headlights, rear screen heater and the blower at full. The voltage may sag to 13.2–13.5 at most and must stay there. Below 13 means the alternator cannot carry the load.
3. Raise the revs to 2000. On a healthy system the voltage barely moves. Rising with the revs means the regulator is not holding.
4. Above 15 V — the regulator has failed. That is worse than undercharging: the battery boils and bulbs burn out.

The full procedure with numbers, including from cold, is in how to check the battery and alternator, and how to tell one from the other by behaviour is in alternator or battery.

Why these codes arrive in a bunch

A sagging supply is the one fault that logs codes in every module at once. If U0100, U0121 and half a dozen other strangers sit next to P0560, do not repair five systems: fix the supply and clear the list. Whatever comes back alone is the real one.

The same goes for lamps: the battery lamp together with ABS and ESP at once is not three faults but one voltage.

What to check by hand

1. Terminals: corrosion, play, green deposits. Tighten and clean.
2. The engine ground — the thick strap from the engine to the body. This is the cause most often skipped.
3. The alternator belt: a squeal when the load comes on means it is slipping.
4. The age of the battery. After five years it no longer holds, and the alternator works harder trying to top it up.

⚠️ If the voltage jumps and the headlights dim in time with the revs, do not put it off. Swings kill electronics that cost more than an alternator.