P21AC: Reductant Level Sensor B Circuit Intermittent/Erratic

P21AC reports that the level sensor B signal drops out or jumps for short moments instead of failing steadily. No manufacturer documents criteria for this code publicly. GM's Duramax and Cruze diesel tables cover sensor 2 only with the steady faults P21AA and P21AB, both requiring an error lasting over 3 s, so an intermittent code points to contact losses shorter than that: a terminal that has lost its grip in the tank connector, a harness section that moves with the tank, or corrosion where fluid has crept in. GM's bulletin PIP4887B on related level sensor codes stresses checking terminal tension even when codes are only stored in history. CARB requires input circuits to be watched for such faults (13 CCR 1968.2 (f)(15.2.1)).

Symptoms

• DEF warning that comes and goes
• Jumping DEF level display
• Check engine light, sometimes only stored code

Most likely causes

• Loose terminal in the tank connector
• Harness flexing or rubbing near the tank mounting
• Corrosion from fluid creeping into the connector

What to check, in order

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

1. Check the AdBlue/DEF level and quality, and do not add additives or water
2. Gently move the wiring with the engine running and watch whether the reading changes.
3. Look for white crystals around the injector and the tank
4. Check connectors and heaters with the scanner
5. Run the dosing test available on your diagnostic tool
6. Replace parts only when the test points to that component

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P21AC means

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

Frequently asked questions

Can I keep driving with code P21AC?

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

What causes code P21AC?

The most common causes are: loose terminal in the tank connector; harness flexing or rubbing near the tank mounting; corrosion from fluid creeping into the connector.

Related codes

• P21A8Reductant Level Sensor B Circuit
• P21AAReductant Level Sensor B Circuit Low
• P21ABReductant Level Sensor B Circuit High
• P203EReductant Level Sensor A Circuit Intermittent/Erratic
• P21B1Reductant Level Sensor C Circuit Intermittent/Erratic

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 14OBDG04 Engine Diagnostics
• static.nhtsa.gov: MC 10112047 9999

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.