P21AB: Reductant Level Sensor B Circuit High

What it means

The control module detects a high signal in the reductant level sensor B circuit. The code alone does not confirm an open circuit or a short to battery; the cause depends on the circuit design.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

Reductant level sensor B reports fluid level information from the DEF/AdBlue tank to the control module. The module monitors sensor B's circuit signal and sets P21AB when that signal is higher than its expected range; the code does not establish whether the signal wire is open or affected by another circuit fault.

Common causes

Most likely first.

1. Open signal line
2. Connector pins corroded - high contact resistance
3. Supply wire pinched against B+ source

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Record freeze-frame data and compare the reported sensor B level with the actual DEF/AdBlue level and any available sensor readings.
2. Inspect sensor B's connector and harness for corrosion, loose pins, damage, or a wire pinched against a battery-positive supply.
3. Check the connector pins for corrosion or poor contact that could add resistance to sensor B's signal path.
4. With the wiring diagram, measure sensor B's signal and supply circuits under the conditions that set the code, comparing readings with the manufacturer's specification.
5. If wiring and connections check out, test sensor B and its ground or supply as applicable, then confirm the reported level responds correctly.

Common fixes

• Clean and secure corroded or loose sensor B connector pins.
• Repair an open or damaged sensor B signal wire.
• Repair wiring pinched against a battery-positive supply.
• Replace sensor B if its output remains out of specification after circuit faults are ruled out.

Can I keep driving?

You can usually drive briefly if the vehicle runs normally, but check the DEF/AdBlue level and heed any countdown or restart warning. Ignoring the fault can leave the tank level reading unreliable and may lead to a no-start restriction or emissions-test failure.