P21DDReductant Heater A Current Too Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light on
- DEF stays frozen longer in winter, dosing starts late
- DEF warning messages after cold starts
- Likely causes
- LikelyTank heater element burnt open
- LikelyWater and corrosion in the pump and heater connector (Ford C3613)
- PossibleMissing heater supply due to relay, fuse or wiring fault
- RareHeater driver in the control module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,800
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.