Powertrain, generic (extended)

P24CD Reductant Tank Cap Switch Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P24CD means the signal line of the DEF cap switch sits below its valid voltage band. With the usual pull-up input that points to the line being pulled to ground: a short in the harness near the filler neck, a switch with an internal short, or moisture bridging the contacts. Whether the controller then reads the cap as permanently open or closed depends on how the switch is wired. No manufacturer documents P24CD publicly. The same switch appears in Audi's bulletin 2051053 for the Q7 TDI with the general and range codes P24CB and P24CC, which shows where it sits and that it can only be replaced with the tank out. CARB requires input circuits of emission-related components to be monitored for readings outside their valid band. The opposite case is P24CE; sporadic drops are P24CF.

Likely causes

Signal wire chafed through to ground near the filler neck
Moisture or DEF in the switch or connector bridging to ground
Switch with internal short

Symptoms

  • Malfunction indicator lamp on
  • Cap state shown permanently wrong, refills not detected

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant tank cap switch

Related codes

P24CBP24CCP24CEP24CF

References: law.cornell.edu, static.nhtsa.gov.