P2ACE: Reductant Temperature Sensor B Circuit Low

What it means

Voltage on the reductant temperature B sensor signal line is below the expected window. Wire shorted to ground or sensor short.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

Reductant temperature sensor B sends the control module a voltage signal for the temperature of the AdBlue/DEF aftertreatment fluid. This code sets when the signal voltage is below its expected range, often because the signal or supply is shorted to ground or the sensor has failed.

Common causes

Most likely first.

1. Signal wire shorted to ground
2. Sensor fault
3. Short to ground in the sensor supply circuit

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. Save freeze-frame data and check sensor B's live temperature for an implausible low or fixed reading.
2. Inspect the sensor B connector for corrosion, fluid ingress, damaged pins, or a poor fit.
3. Check the sensor B harness for damaged insulation or contact with ground, paying attention to the signal and supply wires.
4. With the circuit isolated as specified in the vehicle wiring information, test for a short to ground in the signal and supply circuits.
5. Test sensor B and its wiring against the manufacturer's specification to distinguish a sensor fault from a circuit short.

Common fixes

• Clean and secure corroded or loose sensor B connector terminals.
• Repair the signal or supply wire where it is shorted to ground.
• Replace reductant temperature sensor B if it fails the specified test.

Can I keep driving?

Driving may be possible initially, but the emissions system may show a NOx warning or start a countdown that can restrict operation or prevent restarting. Address the fault promptly and do not ignore a displayed countdown.