P21CDReductant Quality Module Supply Voltage Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- The DEF quality module measures the urea concentration in the tank and, on many tanks, shares a housing with the level sensor. P21CD reports that its supply voltage is too low. Ford's 2023 diesel summary sets the code when the quality sensor supply falls below 10 V or rises above 20 V for 15 s, so on Ford the overvoltage case shares this code. Ford bulletin 19-2315 covers 2017 to 2019 Super Duty trucks with the 6.7 L diesel built up to 1 August 2018: water enters the four-pin connector C4851 at the level and quality module, corroding pins 1 and 4, and P21CD appears together with U02A2. The fix is a new tank harness, sometimes the tank. Without a valid quality reading the SCR system cannot verify the fluid, and P206B to P206D cannot run.
- What you may notice
- Check engine light on
- DEF quality or level warning in the instrument cluster
- Communication code for the quality module stored as well
- Likely causes
- LikelyWater and corrosion in the quality module connector (Ford C4851)
- PossibleDamaged tank harness or supply wire
- PossibleLow battery or charging voltage
- RareQuality module internally 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: €50 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.