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Diagnostic trouble code P21B1 indicates: Reductor (AdBlue) Nivel Sensor C Circuito Intermitente/Errático. It was set by your vehicle's onboard diagnostics system and will typically keep the check engine light on until it's addressed.
Upload your ECU file and DTC Doctor will detect this code (and any others) automatically, then remove it for good — no dealer visit, no guesswork.
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