Cause
Reductant tank heater stuck on
The tank heater energised continuously rather than only below freezing. The heater exists because reductant freezes at around minus eleven degrees, and it should run only when needed.
Running permanently overheats the fluid, which degrades it: concentration drifts out of specification and the quality sensor eventually reports it. So a stuck heater produces a fluid quality fault, and replacing the fluid without fixing the heater degrades the new fluid too. A tank noticeably warm in mild weather is the physical sign. A welded relay is the commonest cause, and it is testable by substitution; a module driver failed short is the other, and both leave the circuit energised with the ignition off, which also drains the battery.
Typically a moderate job.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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