P221A: NOx Sensor 1/2 Correlation (Bank 1)

P221A compares the two NOx sensors of bank 1, upstream and downstream of the SCR catalyst. When conditions should make both read alike, the gap between them is too large, so the system cannot tell which value to trust. Volvo and Mack list P221A among codes that often lead to needless sensor swaps; before replacement a NOx conversion test must show the inlet and outlet readings disagreeing by more than about 40 ppm shortly before engine shutdown, or the cause lies elsewhere. Nissan bulletin NTB16-048 covers early 2016 Titan XD trucks with the Cummins 5.0L V8: P221A with P229E and P22A7 is fixed by a new outlet sensor and ECM reprogramming. P2201 and P229F judge each sensor alone, P221B is bank 2.

Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank.

Symptoms

• Malfunction indicator lamp on
• Often stored with P20EE or other SCR codes
• Implausible NOx conversion values in live data

Most likely causes

• One of the two NOx sensors (often the outlet sensor) reading wrongly
• Exhaust leak between the two sensors diluting the reading
• ECM software (Nissan Titan XD 5.0L 2016, NTB16-048)
• Faulty DEF dosing 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 P221A means

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

Frequently asked questions

Can I keep driving with code P221A?

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 P221A?

The most common causes are: one of the two NOx sensors (often the outlet sensor) reading wrongly; exhaust leak between the two sensors diluting the reading; eCM software (Nissan Titan XD 5.0L 2016, NTB16-048).

Related codes

• P221BNOx Sensor 1/2 Correlation (Bank 2)
• P2201NOx Sensor Circuit Range/Performance (Bank 1 Sensor 1)
• P229FNOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)
• P229ENOx Sensor Circuit (Bank 1 Sensor 2)
• P22A7NOx Sensor Heater Sense Circuit Range/Performance (Bank 1 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.