Powertrain, generic (extended)

P21DD Reductant Heater A Current Too Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P21DD means heater A of the DEF system, which Ford and GM both assign to the tank heater (circuit 1), draws less current than expected while switched on, so it is likely open or barely heating. GM's 2025 diesel tables set it when heater 1 current stays below 0.75 A for 40 samples of 0.1 s during a positive command, Type A with MIL after one trip; the 2014 tables flagged an open tank heater when its conductance fell to 0.27 or 0.35 S (by table) or less at 11 V or more, evaluated a single time per drive, Type B. Ford's 2023 diesel summary tests once per heating cycle after a cold start: below 3 A or too little heater power. Ford bulletin 19-2315 links P21DD on 2017 to 2019 Super Duty 6.7 L trucks to water in connector C3613 at the pump and heater unit. P214F is the high-current partner.

Likely causes

Tank heater element burnt open
Water and corrosion in the pump and heater connector (Ford C3613)
Missing heater supply due to relay, fuse or wiring fault
Heater driver in the control module faulty

Symptoms

  • Check engine light on
  • DEF stays frozen longer in winter, dosing starts late
  • DEF warning messages after cold starts

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–1,800
Time0.5–3 h

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

Parts involved

reductant tank heater

Related codes

P214FP221CP20B9P20BBP20BCP21C2

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.