Faulty NOx sensor 1 (upstream) or sensor 2 (downstream) due to contamination or age; Wiring damage or corroded connectors between the NOx sensors and the control module; Exhaust leaks upstream of the sensors causing false readings; Deteriorated diesel particulate filter or selective catalytic reduction system affecting sensor correlation; Software calibration issue in the powertrain control module; Contaminated fuel or incorrect diesel exhaust fluid concentration; Failed NOx sensor control module (if separate from the sensor)

DTC codes / P22xx · Fuel and air metering (injector circuits) / P221A

P221ANOx Sensor 1/2 Correlation (Bank 1)

Q: What does code P221A mean?

NOx Sensor 1/2 Correlation (Bank 1). P221A indicates that the powertrain control module has detected a correlation error between the two NOx (nitrogen oxide) sensor signals on bank 1, meaning the upstream and downstream sensor readings do not agree as expected. The computer compares the NOx sensor outputs after the engine has reached operating temperature and sets this code when the discrepancy exceeds a calibrated threshold for a set time.

P221A indicates that the powertrain control module has detected a correlation error between the two NOx (nitrogen oxide) sensor signals on bank 1, meaning the upstream and downstream sensor readings do not agree as expected. The computer compares the NOx sensor outputs after the engine has reached operating temperature and sets this code when the discrepancy exceeds a calibrated threshold for a set time.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaFuel and air metering (injector circuits)

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Reduced engine power or limp mode in some vehicles
Slightly rough idle or hesitation during acceleration
Decreased fuel economy due to altered fuel injection strategy
Possible increase in diesel exhaust fluid consumption
In rare cases, a sulfur or ammonia smell from the exhaust

Common causes, most likely first

Faulty NOx sensor 1 (upstream) or sensor 2 (downstream) due to contamination or age
Wiring damage or corroded connectors between the NOx sensors and the control module
Exhaust leaks upstream of the sensors causing false readings
Deteriorated diesel particulate filter or selective catalytic reduction system affecting sensor correlation
Software calibration issue in the powertrain control module
Contaminated fuel or incorrect diesel exhaust fluid concentration
Failed NOx sensor control module (if separate from the sensor)

How to diagnose and fix it

Perform a visual inspection of the NOx sensor wiring, connectors, and exhaust system for damage or leaks
Use a scan tool to read live data from both NOx sensors and compare readings under various operating conditions
Check for technical service bulletins or software updates for the powertrain control module
Test the NOx sensor circuits for continuity, resistance, and short circuits according to manufacturer specifications
Replace the faulty NOx sensor after confirming which sensor is providing an implausible signal
Clear the diagnostic trouble codes and perform a road test to verify the repair
If the code persists, diagnose the selective catalytic reduction system and diesel particulate filter for efficiency issues

Before replacing both NOx sensors

A correlation code does not automatically mean both sensors are bad; often only one sensor is skewed, and replacing both without testing wastes money.
Exhaust leaks between the two sensors can cause a correlation error that mimics sensor failure, so always inspect the exhaust system first.
Low diesel exhaust fluid quality or a failing DEF injector can alter NOx levels and trigger this code even when the sensors are functioning correctly.
Some vehicles require a relearn or adaptation reset after NOx sensor replacement, and skipping this step will cause the code to return.
The code may be set by a software bug in the control module, so check for updates before condemning hardware.

Can I keep driving with P221A?

You can usually continue driving with this code, but avoid heavy loads or long trips until it is diagnosed, as prolonged driving may lead to reduced engine power or increased emissions. If the check engine light starts flashing or you notice severe performance loss, stop driving and seek professional service.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.