P2202Powertrain
Powertrain · SAE
NOx Sensor B1 — Signal Low
Meaning
The electrical signal from the bank 1 NOx sensor is below the minimum permitted threshold. The ECU detects a voltage that is too low, indicating a short to ground, a failed sensor, or an anomalous measurement condition (NOx actually near zero under conditions that do not justify it).
Common causes
- Short circuit of the NOx sensor B1 signal wire to ground
- Internally failed NOx sensor B1 with output stuck low
- Non-functioning heating element in the NOx sensor (cold sensor = low reading)
- Loose or corroded connector with high contact resistance
- Degraded reference voltage artificially pulling the signal low
Symptoms
- Check engine light on
- Upstream NOx value fixed at zero or minimum on scan tool
- Possible AdBlue over-dosing (ECU compensates for the low signal)
- Potentially high actual NOx emissions going undetected
- Related codes P2200 or P2BAC possible
Severity
Moderate to high: a low NOx signal leads to incorrect SCR calibration; AdBlue over-dosing can damage the SCR catalyst (ammonia slip), while under-dosing causes NOx exceedances.
What to do
- Measure sensor output voltage with a multimeter; if below 0.1 V, investigate the short to ground
- Check the NOx sensor heating element operation (resistance and pre-heat cycle)
- Inspect the connector for oxidation and loose pins
- Check the reference voltage at the sensor connector
- Replace the sensor if the heating element is intact and wiring is correct but signal remains low