What are the common causes of P21DD?

High resistance in the reductant heater A power feed. A failed reductant heater A element drawing too little current. Corrosion at the reductant heater A connector dropping current. A blown or partially open fuse in the reductant heater A circuit.

Read reductant heater A current on a scan tool. Check the reductant heater A fuse and power feed. Measure reductant heater A element resistance. Inspect the reductant heater A connector for corrosion. Replace the reductant heater A if its element is open. Clear the code and retest.

P21DD

Reductant Heater A Current Too Low

System: Fuel and air metering (P code)
Lamp: Check Engine Light
Category: Powertrain - Fuel/Air Metering

What it means

P21DD means Reductant Heater A Current Too Low.

Check these first

High resistance in the reductant heater A power feed
A failed reductant heater A element drawing too little current
Corrosion at the reductant heater A connector dropping current
A blown or partially open fuse in the reductant heater A circuit

What to do next

Read reductant heater A current on a scan tool
Check the reductant heater A fuse and power feed
Measure reductant heater A element resistance
Inspect the reductant heater A connector for corrosion
Replace the reductant heater A if its element is open
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.