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P codepowertrainMedium severity

P229ENOx Sensor Circuit (Bank 1 Sensor 2)

The downstream NOx sensor after the SCR catalyst on bank 1 has an electrical circuit fault, so the emissions system cannot verify NOx reduction.

Definition

On SCR-equipped diesels (and some lean-burn gasoline engines), the bank 1 sensor 2 NOx sensor sits after the SCR catalyst and reports catalyst-out NOx over the CAN bus so the PCM can verify DEF dosing and catalyst efficiency. This code sets when the sensor's power, ground, heater, or communication circuit is faulted or the smart-sensor module reports an internal error. The MIL illuminates and SCR monitoring is suspended; on many diesels an unrepaired NOx sensor fault starts an emissions inducement countdown ending in speed limiting or derate, and the vehicle will fail an emissions inspection.

Common causes

  • Failed NOx sensor or its integrated control module (most common)
  • Damaged, chafed, or corroded wiring and connectors to the sensor
  • Blown fuse on the sensor module's battery feed
  • Water intrusion at the sensor connector or module
  • CAN bus communication fault between the sensor module and PCM
  • Cracked sensor element from exhaust condensation (thermal shock)

Common misdiagnoses

  • SCR catalyst replaced for what is a sensor circuit fault
  • DEF injector or DEF pump replaced unnecessarily
  • Upstream (sensor 1) NOx sensor replaced instead of the downstream unit
  • DEF quality blamed and fluid repeatedly drained and refilled

Troubleshooting steps

  1. 1.Scan all modules for related codes

    Accompanying U-codes point to a CAN wiring problem, while a lone P229E with internal-fault status points to the sensor module itself.

  2. 2.Check the sensor module's fuse and power feed

    These smart sensors have a fused battery supply; verify battery voltage and a good ground at the module connector with the key on.

  3. 3.Inspect wiring and connectors along the exhaust

    Look for heat damage, chafing at hangers, and green corrosion or moisture inside the connector, which is a frequent failure point.

  4. 4.Verify CAN communication

    Confirm the sensor appears on the network with a scan tool and check CAN circuit resistance and integrity if it does not.

  5. 5.Replace the sensor as a complete assembly

    The probe and its module are calibrated together and are not serviced separately, so replace the whole unit and clear the fault to stop any inducement countdown.

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