P21AB: Reductant Level Sensor B Circuit High

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.

Medium severity. Can I keep driving?Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

Symptoms

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

Most likely causes

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

What to check, in order

Start with the simplest. You can tick off each step.

Check the AdBlue/DEF level and quality, and do not add additives or water
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Look for white crystals around the injector and the tank
Check connectors and heaters with the scanner
Run the dosing test available on your diagnostic tool
Replace parts only when the test points to that component

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P21AB means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
AB: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21AB?

Deal with it soon. If the instrument cluster shows a countdown, the car may not restart once it reaches zero.

What causes code P21AB?

The most common causes are: open signal wire or spread terminal at the tank connector; signal wire shorted to a voltage source; level sensor B open inside the tank assembly.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.