P0561: System Voltage Unstable
P0561 means a control module has detected that its supply voltage is unstable, or that its two power inputs disagree. It is usually not urgent on its own, but the cause is often a bad connection or fuse that can lead to other electrical problems.
You can drive, but get it checked soon
The reference lists no warning lamp (except on the GM transmission module), no limp mode and no emissions relevance for this code. The cause can be a loose connection or a fluctuating regulator, though, and that can cause electrical dropouts or other voltage codes, so get it checked soon.
What it means
A control module measures the voltage it receives from the car's electrical system. P0561 is stored when that voltage fluctuates, or, in GM vehicles, when the module's two supply inputs (the permanent battery feed and the switched ignition feed) differ by more than 3 V. GM's engine module sets it after 50 of 63 voltage samples, without turning on a warning lamp. GM's 9-speed transmission module sets it after one trip and turns on the lamp. Ford's PCM (the engine computer) can also set it without a warning lamp. The code points to a problem in the power supply to the module, not to a failed engine part.
Likely causes
- Fuse, relay or line of the ignition supply (voltage drop)Very likely
A weak fuse, relay or wire in the ignition feed lowers the voltage at one module input, so the two inputs no longer match.
- Loose or corroded connector or ground at the modulePossible
Corrosion or a loose pin adds resistance, which makes the voltage the module sees jump around.
- Voltage regulator fluctuatingPossible
If the regulator does not hold charging voltage steady, the module records an unstable supply.
- Battery terminal loosePossible
A loose terminal interrupts current briefly and causes voltage swings across the whole system.
What you may notice
- Further voltage or communication codes
- Transmission warning or check engine light (GM transmission module)
- Sporadic dropouts of electrical functions
What to check, cheapest first
- 1Inspect both battery terminals and make sure they are tight and free of corrosion. · you can do it
- 2Check the fuses and relays in the ignition supply circuit for damage or poor contact. · you can do it
- 3Look at the module connector and ground points for looseness, corrosion or moisture. · you can do it
- 4Read all stored codes, including related ones such as P0560, P0562, P0563 and P0685, to see the overall pattern. · mechanic
- 5Measure battery voltage and charging voltage with the engine running to test the regulator. · mechanic
- 6Measure voltage at both supply inputs of the module and look for a voltage drop along the ignition feed line. · mechanic
Related codes
Frequently asked questions
Can I drive with P0561?+
Usually yes for a short time, with care. The code does not normally cause limp mode, but an unstable supply can lead to electrical dropouts, so have it checked soon.
Can a loose battery terminal cause P0561?+
Yes. A loose battery terminal is a listed cause because it makes system voltage fluctuate. It is also one of the cheapest things to check.
Will P0561 fail an emissions test?+
The reference marks this code as not emissions relevant and it normally sets no warning lamp, except on the GM transmission module. A lit check engine light can still cause a failed inspection in some areas.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10