P21AF: Reductant Level Sensor C Circuit Low

What does P21AF mean?

P21AF is set when level sensor C, called level sensor 3 by GM, reads below 0.17 V right after the tank electronics apply their test pulse, which GM interprets as a short to ground. Supply must be above 8 V and the error must persist for more than 3 s. The 2014 Cruze diesel stores it as Type B, so the MIL lights after the second failing trip; on the 6.6L LML Duramax it appears in the same form, and GM bulletin 12-06-04-002C places it in the first group of DEF codes to be repaired before the warning tests can run again. When P203C or P21AA are present as well, several sensors reading low at once point to a common wiring or connector problem rather than to sensor 3 alone.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light after two trips
• DEF warning message
• Implausible DEF level shown

Common causes

• High Sensor 3 signal wire shorted to ground
• Medium Water or fluid bridging contacts in the tank connector
• Medium Level sensor C shorted inside the tank assembly

Likelihood is a general estimate across vehicles; your make and model may differ.