U0251: Lost Communication With Running Board Control Module B
What to do
The car drives normally; the powered step on one side has stopped.
Check what the other side is doing, and tell whoever looks at it. If both boards have stopped, the fault is something they share — a fuse or a supply — which is one repair. If only one has, it is that side's own wiring or mechanism.
Watch for the awkward combination of one board out and one in. The extended one adds several inches of width and will catch kerbs and bollards, while the retracted one leaves a high step nobody expects. Warn passengers on the side without the step, particularly children and anyone less mobile.
If this side has failed before, mention that too. The kerb side gets used far more and takes all the gutter water, salt and kerb strikes, so it fails more often — and fitting another identical part without protecting it better will produce the same result again.
What this code means
The second running board control module has stopped being heard from — the other side of the pair described at U0244, where the stuck-extended and stuck-retracted consequences are set out.
A pair of powered boards is the normal arrangement, one per side, and the comparison they allow is the fastest diagnostic step available.
Both silent points at what they share — a fuse, a supply, the body control module, or the network branch. One silent points at that side's own module, motor, wiring or mechanism.
And on this component the asymmetry is usually explicable, which makes it worth asking about. The kerb-side board takes the traffic: it is the one used every time somebody gets in or out on a pavement, the one exposed to gutter water and salt spray, and the one that catches kerbs when parking. The offside board on a commuter's vehicle may be used a fraction as often. A repeated failure on one side is therefore usually about duty and environment rather than a bad part — and the same replacement will fail the same way.
One practical point about a pair that does not apply to a single unit: the two need to agree. A vehicle with one board extended and one retracted looks odd and is worse than that — the extended side is adding width and catching obstacles while the retracted side offers no step, so the driver gets both problems at once and neither is expected.
Causes are as at U0244: this is the lowest-mounted, most exposed module on the vehicle, and corrosion and impact dominate.
Likely causes
Start here
- Water ingress into the module connector · moderate
- Physical damage from road debris · moderate
- Corroded or loose connector · easy
- Damaged wiring harness · moderate
Less common
- Blown fuse or failed supply · easy
Symptoms
Components involved
Information, not instruction
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Community experience
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