P221A

Code

Generic P — Powertrain

NOx Sensor 1/2 Correlation Bank 1

Brand: Generic

Type: P — Powertrain

Views: UK: 24 EN: 54 RU: 24

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Causes

Faulty NOx sensor (upstream or downstream) on Bank 1
Open, shorted, or intermittent wiring or poor connector connection to one or both NOx sensors
Exhaust leak between the two sensor locations (manifold, downpipe, flange)
Contaminated sensor due to oil, coolant, or soot
Heater circuit failure in one of the NOx sensors (if equipped)
Aftertreatment hardware issue (DPF, SCR catalyst, or injector malfunction) changing NOx levels

Symptoms

Check Engine Light (MIL) illuminated
Reduced or altered engine performance or default limp mode in some vehicles
Failed emissions test or high NOx readings on tailpipe inspection
Possible increased fuel consumption if ECU modifies strategy
Diagnostic trouble codes related to NOx/aftertreatment

What to check

Read and record freeze-frame data and all pending/active codes with a diagnostic scan tool supporting live NOx sensor data
Compare live NOx sensor 1 and sensor 2 readings (ppm or voltage/current) at warm idle and during steady cruise
Inspect exhaust system for leaks or damaged flanges/pipes between the two sensors
Visually inspect sensor connectors and wiring harness for corrosion, damage, chafing, or burns
Check sensor heater supply and ground (resistance or power supply) if sensor includes heater
Check for contamination (oil, coolant, soot) on sensor tips when removed

Signal parameters

NOx sensor output: typically a low-voltage signal or current proportional to NOx concentration (common ranges: 0–5 V or 0–20 mA depending on sensor type)
Expected correlation: Sensor 1 and Sensor 2 should track closely once engine and aftertreatment are at operating temperature (within a small percentage or defined ppm threshold)
Heater resistance: compare measured heater resistance to manufacturer spec (often tens of ohms; varies by sensor)
Warm-up behaviour: sensors should reach operating temperature within specified time and show responsive changes to throttle/load
CAN or serial NOx data: check that reported NOx ppm for both sensors are present and updating on the bus

Diagnostic algorithm

Use a scan tool to confirm P221A is current and capture freeze-frame and live NOx sensor data. Note the operating conditions when the code set (engine temperature, load, speed).
Verify basic electrical integrity: disconnect the battery if required by service procedures and inspect connectors for corrosion, bent pins, or water ingress. Reconnect and re-scan.
With the engine at normal operating temperature, monitor live NOx readings for Sensor 1 and Sensor 2 during idle and steady cruise. Look for large differences, delayed response, or one sensor stuck/flatlining.
Visually inspect exhaust components between the two sensors for leaks, cracks, or missing gaskets. Repair any leaks and retest.
Check sensor heater circuit (if present): measure voltage at connector during key-on/start and measure heater resistance per service spec. Replace sensor if heater open or out of range.
Perform wiring checks: backprobe sensor signals and grounds to verify continuity and absence of short to power or ground. Wiggle test harness while monitoring live data to find intermittent faults.
If wiring and connectors are good, swap sensors between positions only if permitted by manufacturer guidance and if sensors are identical. If the code follows the sensor, replace the sensor. If the code stays with the position, suspect wiring/exhaust or ECM.
Inspect sensor tips for contamination when removed. Replace contaminated sensors and address root cause (oil leak, EGR/coolant leak, excessive soot) before fitting new sensors.
Check for related aftertreatment faults (DPF, SCR injector, dosing system) that could create abnormal NOx levels and correct those first.
After repairs, clear codes and perform a drive cycle to confirm sensor correlation is restored and P221A does not return. If code persists, consult factory service information and consider ECM diagnostics or reflashing.

Likely causes

Damaged wiring or poor connector at NOx Sensor 2 (most common)
Failed NOx sensor (one sensor reading drifted or stuck)
Exhaust leak between Sensor 1 and Sensor 2 causing different NOx concentrations
Contaminated sensor from oil/soot causing slow or incorrect response

Fault status

⚠️ Status

ECM detected an out-of-range correlation between NOx Sensor 1 and NOx Sensor 2 on Bank 1 — sensors' readings or response characteristics do not match within allowed limits.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours