P2221NOx Sensor Heater Sense Circuit (Bank 2)
There is a general fault in the circuit the control module uses to monitor (sense) the Bank 2 NOx sensor heater's operation.
Definition
The system verifies the Bank 2 NOx sensor heater through a dedicated sense circuit that feeds back heater voltage or current, confirming the element actually heats and regulates. P2221 sets when that feedback is missing or implausible, so the module can no longer trust that the sensor is at operating temperature. NOx data from Bank 2 is discarded, the MIL illuminates, and the NOx/SCR monitors will not complete — failing an emissions inspection and degrading DEF dosing accuracy on diesel applications over time.
Common causes
- Internal NOx sensor fault in the heater feedback circuitry
- Open or shorted heater sense wire
- Corroded or loose connector terminals
- Water intrusion into the sensor module
- Control module input circuit fault (rare)
Common misdiagnoses
- SCR catalyst
- DEF pump or injector
- Bank 1 NOx sensor
- Fuses/relays replaced without finding the sense-circuit fault
Troubleshooting steps
1.Check for companion heater codes
If Bank 2 heater control codes (P2218-P2220) are also set, diagnose shared power and ground first; an isolated P2221 targets the sense line or sensor.
2.Test sense wire continuity and isolation
Verify the feedback wire reads near 0 ohms end to end and shows no continuity to ground or the heater supply.
3.Verify sensor power and ground under load
Marginal grounds commonly corrupt feedback signals; confirm less than 0.1 V drop on the ground circuit with the heater operating.
4.Compare commanded vs. reported heater data
On the scan tool, feedback frozen at zero or full scale while the heater is commanded confirms the sense fault.
5.Replace the NOx sensor if wiring checks pass
The feedback circuitry is inside the smart sensor module, so a verified harness means the sensor assembly has failed.