What it means
The control module monitors current draw in reductant heater A and sets this code when the draw falls below the specified threshold. The code indicates an electrical current problem in that heater circuit; it does not by itself identify the failed part.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater A warms the diesel exhaust fluid (DEF/AdBlue) so it can flow when cold. The control module monitors current in heater A’s circuit and sets P21DD when the draw is below its expected threshold, which can mean an open circuit, poor connection, or failed heater.
Common causes
Most likely first.
- Open or damaged heater wiring
- Loose or corroded connector
- Faulty reductant heater element
- Blown fuse or poor power/ground connection
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
- Record freeze-frame data and check for related reductant-system codes to see when the low-current fault occurred and whether other heater circuits are affected.
- Inspect heater A’s wiring and connector for breaks, chafing, moisture, corrosion, or loose pins, paying particular attention to sections exposed to road spray.
- Check the relevant fuse and verify the heater circuit’s power and ground connections against the manufacturer’s wiring information.
- With the circuit safely isolated as specified by the manufacturer, check heater A’s wiring continuity and look for an open or high-resistance connection.
- Measure heater A’s element resistance and compare it with the manufacturer’s specification to determine whether the heater itself is open or out of range.
Common fixes
- Clean and secure a loose or corroded heater A connector.
- Repair damaged or open wiring in the heater A circuit.
- Restore a poor power or ground connection, or replace a blown fuse after checking for the cause.
- Replace reductant heater A if its element fails the manufacturer’s resistance check.
Can I keep driving?
The vehicle may remain driveable initially, but the DEF system may not thaw or dose correctly in cold conditions, increasing NOx emissions. Heed any AdBlue/DEF warning or no-start countdown and arrange repair promptly.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.