P0561
System Voltage Unstable
The ECU monitors battery voltage and detects unstable behaviour - rapid fluctuations beyond what current load changes explain. Typical causes are a failing alternator, loose battery terminals, weak battery, or a heavy load (radiator fan, glow plugs, heated screen) drawing voltage down briefly.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Alternator regulator failingCommon
- Battery weak or terminals loosePossible
Parts involved
- Alternator
- Battery
- Main Supply Wiring
P0561 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.