Cause
Stuck sensor float
A float-type level sensor whose float has jammed, so it reports a level that no longer changes. The reading stays perfectly plausible, which is why this fails quietly: the module sees a valid value in range and has no reason to doubt it, and the driver sees a gauge that simply stops moving.
A float stuck high is the dangerous direction — a reductant or fuel tank reported full while it empties, so the first warning is a system fault or a stall rather than a low-level light. Stuck low triggers warnings and derates on a tank that is genuinely fine, which is annoying rather than damaging but sends people to refill a full tank.
What jams it says something about the fluid. Crystallised reductant deposits, debris in a fuel tank, or a float arm bent during a pump or sender replacement. Watch the reported level across a refill rather than at one moment — a value that does not track fluid going in is the finding, and no static test at a single level will show it.
Typically a moderate job.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
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