P221CReductant Heater B 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
- P221C reports that heater B of the DEF system draws too little current while switched on. GM and Ford both assign B to the heated pressure line that carries DEF to the dosing valve. GM's 2025 Duramax tables flag line heater current under 0.75 A with a PWM command above zero, 40 faulty readings spaced 0.1 s apart, Type A with the MIL after one trip. Ford's 2023 diesel summary watches heater B continuously while it runs and decides within 2.2 s: below 3 A with more than 5 V supplied, line power below 1 W or lower than expected. GM's 2014 tables for the 6.6L LML judged conductance instead (0.28 S or less), Type B. An open or burnt line heater is the usual find; P221D is the high-current partner.
- What you may notice
- Malfunction indicator lamp on
- DEF line may freeze in winter, dosing starts late
- DEF system warning, inducement possible if unresolved
- Likely causes
- LikelyHeating element in the DEF pressure line open or burnt
- PossibleCorroded or loose connector at the heated line
- RareMissing heater supply (fuse, relay or harness)
- 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: €100 – €700
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.