P221B: NOx Sensor 1/2 Correlation (Bank 2)

P221B compares the NOx sensors before and after the SCR catalyst of bank 2. When operating conditions should make them agree, their readings differ by more than the calibration allows, and the controller cannot decide which value is wrong. The code requires a separate SCR line on bank 2, which no publicly documented diesel has; no manufacturer lists P221B. The bank 1 twin P221A shows the practice: Volvo and Mack require a NOx conversion test before any part is changed and only accept a sensor fault if inlet and outlet readings differ by more than roughly 40 ppm shortly before shutdown, while Nissan bulletin NTB16-048 replaced the outlet sensor and reflashed the ECM on 2016 Titan XD trucks.

Bank 2 is the side without cylinder 1, so it only exists on V or boxer engines.

Symptoms

• Malfunction indicator lamp on
• Often stored with SCR efficiency codes
• Inlet and outlet NOx values for bank 2 disagree in live data

Most likely causes

• One of the two bank 2 NOx sensors reading wrongly
• Exhaust leak between the two sensors
• Dosing that does not stop or leaks, distorting the comparison

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes and the NOx data before and after the SCR catalyst
2. Compare both signals at the same time in live data and check any recent installation work.
3. Check the sensor's connector and wiring
4. Look for exhaust leaks ahead of the sensor
5. Confirm that the AdBlue/DEF is of good quality
6. Replace the sensor only after ruling out power, ground, and leaks

Good to know

• Repair difficulty: hard
• It affects emissions.

What each character of P221B means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 2: Fuel injection
• 1B: Specific fault in that system

Frequently asked questions

Can I keep driving with code P221B?

You can usually keep driving in the short term, but get it repaired soon: on many diesels, the warning eventually limits starting or engine power.

What causes code P221B?

The most common causes are: one of the two bank 2 NOx sensors reading wrongly; exhaust leak between the two sensors; dosing that does not stop or leaks, distorting the comparison.

Related codes

• P221ANOx Sensor 1/2 Correlation (Bank 1)
• P2214NOx Sensor Circuit Range/Performance (Bank 2)
• P2213NOx Sensor Circuit (Bank 2)
• P220DNOx Sensor Supply Voltage Circuit (Bank 2 Sensor 2)
• P20EESCR NOx Catalyst Efficiency Below Threshold (Bank 1)

Sources

• static.nhtsa.gov: MC 10138731 9999
• static.nhtsa.gov: MC 10188078 0001
• static.nhtsa.gov: MC 10192818 9999
• law.cornell.edu: 13 CCR 1968.2

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.