What it means
The reductant control module supply voltage is below the calibrated limit. The code does not establish where in the supply circuit the voltage drop occurs.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant control module relies on its supply voltage to operate DEF dosing and related emissions functions. P21CB sets when the monitored supply falls below its calibrated limit; it identifies low voltage at the module but not where the voltage drop originates.
Common causes
Most likely first.
- Weak battery or low system voltage
- Voltage drop in module power wiring
- Poor battery or ground connection
- Blown or high resistance fuse
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
- Capture freeze-frame data and check battery, charging, and reductant-system codes to determine whether low voltage affected other modules too.
- Check battery condition and charging-system operation, especially if other low-voltage warnings or codes are present.
- Inspect the reductant module's power and ground wiring, connectors, and fuse for looseness, corrosion, heat damage, or moisture.
- Measure voltage at the module supply and ground while the fault conditions are present, comparing readings with the manufacturer's specification.
- If voltage is low at the module, perform voltage-drop and continuity checks along the fuse, power wire, and ground path to find the resistance or open circuit.
Common fixes
- Charge or replace a weak battery as needed.
- Clean and tighten poor battery or module ground connections.
- Replace a blown fuse after identifying why it failed.
- Repair a damaged or high-resistance power wire or connector.
Can I keep driving?
The vehicle may run, but the DEF system can become unavailable and lead to a no-start countdown after the ignore period. Avoid long or unnecessary trips until the low supply is corrected, since the emissions warning may progress to a restart restriction.
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.