P21CD: Reductant Quality Module Supply Voltage Low
Standardized SAE J2012 code · P21xx — Fuel & air metering and auxiliary emission controls
What does P21CD mean?
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.
The code tells you which monitor failed, not which part is broken. Test before replacing parts.
Symptoms
- Check engine light on
- DEF quality or level warning in the instrument cluster
- Communication code for the quality module stored as well
Common causes
- High Water and corrosion in the quality module connector (Ford C4851)
- Medium Damaged tank harness or supply wire
- Medium Low battery or charging voltage
- Low Quality module internally faulty
Likelihood is a general estimate across vehicles; your make and model may differ.
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