P229FPowertrain
Powertrain · SAE
NOx Sensor Bank 1, Sensor 2 — Circuit
Meaning
The ECU has detected a fault in the circuit of the NOx sensor on bank 1, position 2 (downstream, after the SCR catalyst). The sensor monitors NOx reduction effectiveness and its signal is invalid or outside the permitted range.
Common causes
- Damaged, loose, or oxidised connector or wiring on NOx sensor B1S2
- Internally failed NOx sensor B1S2
- Missing supply voltage or ground at the sensor
- NOx sensor deteriorated by AdBlue crystal contamination
- Short circuit or open circuit in the signal line
Symptoms
- Check engine light on
- ECU unable to monitor SCR catalyst efficiency
- Possible emissions limp mode activation
- No downstream NOx data available on scan tool
- Related SCR/AdBlue codes possible (P207F, P2BAC)
Severity
Moderate to high: without the downstream NOx sensor the ECU cannot verify SCR efficiency and may limit vehicle power or trigger emission protection strategies.
What to do
- Inspect the NOx sensor B1S2 connector and wiring for damage, oxidation, or looseness
- Check supply voltage and ground at the sensor (typically 12 V and 5 V reference)
- Inspect the sensor probe for AdBlue crystal deposits around the mounting area
- Measure sensor resistance and compare with manufacturer specification
- Replace the sensor if wiring is intact but signal is absent or out of range