P21CE

Code

Generic P — Powertrain

Reductant Quality Module Performance

Brand: Generic

Type: P — Powertrain

Views: UK: 27 EN: 45 RU: 28

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Page language: EN

Causes

Defective reductant quality sensor or dedicated quality module
Contaminated, diluted, or incorrect reductant (DEF/AdBlue) fluid
Crystallization or deposits in lines, sensor or dosing system
Open/short/intermittent wiring or poor connector at sensor/module
Heater element failure in quality sensor (if equipped)
Low reductant level or air ingestion in supply

Symptoms

Malfunction Indicator Lamp (MIL)/check engine light illuminated
Aftertreatment/DEF warning message on dash
Reduced engine power or emissions-related derate
Failure to allow normal SCR dosing or inhibited regeneration
Stored and recurring P21CE and possibly related SCR/DEF codes
Visible crystallized deposits or abnormal DEF appearance/odour

What to check

Read all stored DTCs and freeze-frame data; note conditions when fault set
Scan live data: reductant quality sensor values, temperature, level, dosing rate, pump current, and related PID values
Visually inspect DEF tank, lines, filters and sensor for contamination or crystallization
Check DEF fluid: level, fill history, look for discoloration, particulates or foreign liquids
Inspect wiring and connectors for corrosion, pin damage, loose terminals, water intrusion, or chafing
Check sensor/module power and ground voltages and CAN/communication integrity

Signal parameters

Reductant quality sensor output signal (voltage/current/resistance) — should respond to known-good fluid and temperature changes
Sensor temperature reading (if available) — heater should maintain expected range at low ambient temperatures
DEF level percentage and tank temperature PIDs
Dosing command vs actual dosing flow or injector current
Pump voltage and current draw under commanded operation
CAN/serial messages between reductant quality module and powertrain ECU (message presence and update rate)

Diagnostic algorithm

Retrieve codes and freeze-frame; record related DEF/SCR PIDs and conditions when DTC set.
Visually inspect DEF tank, cap, sensor area, lines and electrical connectors for contamination, damage or corrosion.
Check DEF quality visually and by smell; if uncertain, sample fluid for lab/known-good comparison or replace with fresh manufacturer-spec fluid.
Check wiring: backprobe sensor connector, verify proper power, ground and communication signals with ignition on and engine running as required.
Measure sensor outputs and heater resistance (if equipped) and compare to manufacturer values or verify responsiveness by warming sensor or adding known-good fluid.
Command pump/doser and observe pump current, pressure/flow and dosing feedback PIDs to confirm system functionality.
If wiring and supply are good but sensor output is out of spec, replace the reductant quality sensor/module. After replacement, perform required calibrations/relearn procedures.
Update ECU/module software if a known software issue or TSB applies.
Clear codes and perform road/operational test to confirm the fault does not return; monitor DEF system behavior under various temperatures and loads.
If intermittent, consider harness repair or module replacement and verify CAN bus integrity and termination.

Likely causes

Failed quality sensor or internal module electronics
Incorrect or contaminated DEF (water, oil, diesel or other fluids)
Damaged wiring harness or corroded connector at the sensor
Sensor heater not working, causing out-of-spec readings at low temp
DEF crystallization blocking sensor exposure or dosing lines
Intermittent CAN bus fault or module power supply problem

Fault status

⚠️ Status

Reductant Quality Module Performance fault detected. Possible sensor or module failure, poor/contaminated reductant, wiring/connectivity issue, or communication/calibration problem. Emissions control function may be degraded and fault present in memory.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-3.0 hours