P21AB: Reductant Level Sensor B Circuit High

What does P21AB mean?

P21AB is the high-side counterpart: GM sets it when level sensor 2 shows an open load (between 3.56 and 4.74 V, read 1.5 ms after the test pulse) or a short to battery voltage (above 4.74 V), each for more than 3 s, as Type A on the 6.6L LML and the 2014 Cruze diesel. GM's preliminary bulletin PIP4887B for 2010–2011 Silverado, Sierra, Express and Savana with the LML or LGH engine names P21AB together with P203D and P21B0 and the message Service Exhaust Fluid System: inspect wiring, connectors and terminal tension also when the faults are merely stored, and replace the DEF tank only when the wiring is sound. For the 2011 model year GM sorted P21AB into its tamper-related group.

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

Symptoms

• Check engine light on at once
• Service Exhaust Fluid System message
• Several level sensor codes stored together

Common causes

• High Open signal wire or spread terminal at the tank connector
• Medium Signal wire shorted to a voltage source
• Medium Level sensor B open inside the tank assembly

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