P21DD: Reductant Heater A Current Too Low
Standardized SAE J2012 code · P21xx — Fuel & air metering and auxiliary emission controls
What does P21DD mean?
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.
The code tells you which monitor failed, not which part is broken. Test before replacing parts.
Symptoms
- Check engine light on
- DEF stays frozen longer in winter, dosing starts late
- DEF warning messages after cold starts
Common causes
- High Tank heater element burnt open
- High Water and corrosion in the pump and heater connector (Ford C3613)
- Medium Missing heater supply due to relay, fuse or wiring fault
- Low Heater driver in the control module faulty
Likelihood is a general estimate across vehicles; your make and model may differ.
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