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)).

Likely causes

Symptoms

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

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: law.cornell.edu, gsi.ext.gm.com, static.nhtsa.gov.