P2BAE

Code

Generic P — Powertrain

NOx Exceedence - NOx control monitoring system

Brand: Generic

Type: P — Powertrain

Views: UK: 36 EN: 406 RU: 39

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

NOx sensor drift, contamination or failure
Faulty NOx sensor heater circuit or wiring/connectors
Faulty DEF/AdBlue dosing injector (stuck, clogged or electrically faulty)
Low, contaminated or incorrect DEF/AdBlue fluid
Degraded or damaged SCR catalyst or catalyst coating failure
Exhaust leaks upstream of NOx sensors causing incorrect readings

Symptoms

Malfunction Indicator Lamp (MIL) / Check Engine light ON
Aftertreatment/DEF warning lamp or message (if present)
Failed emissions test for NOx or ON-BOARD monitor test failure
Reduced engine performance or limp-home strategy in some vehicles
DEF tank low/quality messages or dosing system warnings
Possible unusual exhaust odors (urea/chemical)

What to check

Read and record all stored and pending DTCs and freeze-frame data
Check vehicle service history for recent repairs to exhaust/aftertreatment/DEF systems
Confirm DEF/AdBlue level, quality (contamination, crystallization) and correct type
Inspect NOx sensor(s) and heater wiring/connectors for corrosion, damage or corrosion at pins
Visually inspect exhaust system for leaks, holes or loose joints upstream of NOx sensors
Check for other related codes (sensor circuits, dosing control, temperature sensors)

Signal parameters

NOx sensor upstream (raw) and downstream (post-SCR) readings (ppm or calibrated units)
Delta NOx (upstream minus downstream) across SCR — should exceed calibrated threshold when system functioning
NOx sensor heater voltage and resistance values
DEF dosing command (ms or pulse duty) vs actual dosing feedback (if available)
SCR catalyst temperature (°C) and exhaust temperature upstream/downstream
DEF tank level sensor, DEF tank temperature, and DEF concentration/quality monitors (if equipped)

Diagnostic algorithm

Retrieve all codes and freeze-frame data; note operating conditions when the fault set (load, speed, temp).
Check DEF level and quality; top up with correct fluid if low and clear contamination if present. Record any DEF tank sensor faults.
Visually inspect NOx sensors and wiring for damage, corrosion, loose connectors, or exhaust leaks upstream that could skew readings.
Measure NOx sensor heater resistance and apply 12V to heater circuit to confirm operation (follow OEM safety procedures).
Monitor live NOx sensor data: compare upstream vs downstream readings during idle and under load. Verify the ECU sees expected delta when dosing occurs.
Command an active DEF dosing event (if service tool available) and observe dosing command, injector response, and post-SCR NOx change. Check injector for clogging or mechanical failure if dosing not observed.
If dosing works but downstream NOx remains high, perform SCR efficiency test: confirm catalyst temperature, perform forced regeneration or SCR self-test per OEM procedure, and measure delta NOx again.
Check related sensors (exhaust temp sensors, O2 sensors if present) and EGR/combustion parameters for root causes of elevated NOx production.
Repair identified faults (clean/replace NOx sensor(s), repair wiring/connectors, replace dosing injector, replace SCR catalyst, or correct DEF quality).
Clear codes, perform required relearns or sensor calibrations, and verify fix via road test and live data; confirm NOx readings stay within calibrated limits.

Likely causes

NOx sensor out-of-spec or intermittent (most common)
DEF dosing injector blocked or inoperative
DEF quality low or tank empty
Wiring harness or connector corrosion/opens affecting sensor or dosing feedback
SCR catalyst inefficiency or poisoning

Fault status

⚠️ Status

NOx exceedence detected by aftertreatment control — NOx emissions exceed calibrated limit. Inspect NOx sensors, DEF dosing, SCR catalyst and associated wiring/consumables.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 2.0-4.0 hours