U02A5

Code

Generic U — Network/User

Lost Communication With Reductant Heater Control Module

Brand: Generic

Type: U — Network/User

Views: UK: 31 EN: 37 RU: 35

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or shorted wiring between ECM/BCM and reductant heater control module
Poor or missing module ground or supply voltage
Faulty reductant heater control module
CAN/LIN network fault (bus short, high resistance, missing termination)
Corroded or disconnected connector(s)
Intermittent wiring due to chafing, heat, or vibration

Symptoms

MIL (malfunction indicator lamp) or DEF warning lamp illuminated
Reductant heater not operating when commanded (no warm-up)
Reduced engine power or limp-home mode related to emissions system
DTCs related to DEF/urea dosing, heater, or CAN communication present
No response to diagnostic communication attempts to that module

What to check

Read all related communication and DEF system DTCs with a capable scan tool
Attempt to communicate with the reductant heater module using OEM-level diagnostics
Visually inspect connectors at the reductant heater module and harness for corrosion, bent pins, or damage
Check module supply fuse(s) and relay(s) for continuity and correct operation
Measure supply voltage and ground at the module connector with ignition on
Check CAN (or LIN) bus continuity, termination resistance, and differential voltages

Signal parameters

CAN High / CAN Low differential voltage (idle ~2.5 V, differential ~0 V; active ±1 V swings)
Bus dominant/recessive waveform quality and message frequency for reductant heater module ID
Module supply voltage (should be battery voltage with ignition on)
Module ground resistance to chassis (low ohms)
Termination resistance across CAN_H and CAN_L (approx. 60 ohms for two 120Ω terminators)
Diagnostic tool response: module presence/absence and response time

Diagnostic algorithm

Record freeze frame and all related DTCs. Note conditions when code set (ignition state, temperature, etc.).
Use a scan tool to attempt to address the reductant heater module. If module does not respond, confirm code is current and not pending.
Visually inspect module connectors and harness for corrosion, pin damage, water intrusion, or heat damage. Repair or clean as required.
Check fuses and relays that feed the reductant heater module. Replace any blown fuses and re-test.
With ignition ON, measure battery voltage at module power pin(s) and verify good ground at module ground pin. Repair any supply or ground issues.
Check CAN bus wiring: measure differential voltage, check for proper termination resistance, and look for shorts to battery/ground. Repair wiring faults.
If wiring and power/ground are good, attempt to communicate with module using OEM tool and read module-specific data. If no communication, swap with known-good module only where OEM procedures allow.
If module is non-responsive but other modules show good CAN traffic, consider module replacement. Reprogram/configure new module per OEM procedure.
Clear codes, perform relearn procedures if required, and verify system operation and that DTC does not return after road/operational cycles.

Likely causes

Blown fuse or relay supplying reductant heater module
Connector corrosion/loose pins at module or harness connectors
Damaged CAN bus wiring near module or harness splice
Failed reductant heater control module
Module has gone to sleep or is not powered due to BCM/ECM logic

Fault status

⚠️ Status

Lost Communication With Reductant Heater Control Module — the vehicle network cannot communicate with the DEF heater controller. Emissions control and heater operation may be affected.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours