P2E4C
NOx Sensor Signal Stuck Lean (Bank 1 Sensor 2)P2E4C is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Signal Stuck Lean (Bank 1 Sensor 2). It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P2E4C means
P2E4C is set when the ECM or SCR control module detects that the downstream NOx sensor on Bank 1 (Sensor 2, post-SCR catalyst) is reporting an implausibly low or near-zero NOx concentration for an extended period that does not vary appropriately with changes in engine operating conditions. A downstream NOx reading that remains fixed at a low value regardless of engine load or DEF dosing changes is considered stuck and therefore irrational.
A stuck-lean condition on the downstream NOx sensor can arise from sensor contamination, a failed sensing element that has lost sensitivity, an internal short within the sensor that clamps the output, or a wiring fault that holds the signal at a fixed low voltage. It is also possible for excessive DEF dosing to cause genuine near-zero NOx post-catalyst, but the signal will still respond dynamically in that scenario, distinguishing a real catalyst event from a stuck sensor.
With the downstream sensor stuck, the ECM cannot accurately evaluate SCR catalyst efficiency or adjust dosing strategy based on post-catalyst NOx feedback. This can cause either over-dosing (ammonia slip) or undetected catalyst degradation depending on the control strategy's response to the stuck signal.
Common causes
Most-frequently reported root causes when P2E4C is logged.
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1
Failed NOx sensor element with degraded sensitivity producing a fixed low output.
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2
Sensor contaminated by ammonia slip deposits or DEF crystallization affecting element chemistry.
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3
Short circuit within the sensor generating a constant low-voltage signal.
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4
Wiring fault holding the signal line at a low fixed voltage.
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5
Sensor heater failure causing the element to operate below normal temperature and lose sensitivity.
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6
Physical damage to the sensor from impact or thermal shock.
Symptoms drivers notice
How to diagnose P2E4C
A typical diagnostic flow when this code is present.
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1
Retrieve all DTCs; note any dosing system, upstream NOx sensor, or catalyst efficiency codes.
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2
Monitor downstream NOx sensor live data through a variety of engine load conditions; a healthy sensor should vary with operating mode.
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3
Verify the sensor heater circuit is operating correctly by monitoring heater current on the scan tool.
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4
Inspect the sensor connector and wiring for shorts, corrosion, or damage.
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5
Compare upstream and downstream NOx readings; a persistently stuck downstream value despite varying upstream confirms a sensor fault.
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6
Replace the downstream NOx sensor if live data confirms the signal is non-responsive and wiring checks pass.
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7
Clear codes and retest SCR efficiency monitor after sensor replacement.
Vehicles where we've handled P2E4C
Platforms in our catalogue with confirmed P2E4C coverage.
Related powertrain codes
- P2E34 — Exhaust Gas Recirculation Mass Flow Sensor Circuit Low
- P2E3D — NOx Sensor Heater Control Circuit Low (Bank 2 Sensor 1)
- P2E4D — NOx Sensor Signal Stuck Rich (Bank 1 Sensor 2)
- P2E52 — Reductant Pump Control Circuit Open
- P2E53 — Reductant Pump Control Circuit Low
- P2E54 — Reductant Pump Control Circuit High
Frequently asked questions
Could excessive DEF dosing be confused with a stuck sensor?
In a real over-dosing scenario the downstream NOx signal would still change dynamically with engine load cycles. A genuinely stuck sensor produces a flat, non-responsive trace regardless of conditions.
What is ammonia slip and how is it related to P2E4C?
Ammonia slip occurs when excess DEF is injected and unreacted ammonia exits the tailpipe. If the downstream sensor is stuck low, the control module may continue or increase dosing unnecessarily, risking ammonia slip.
Is P2E4C a sensor or catalyst code?
It is a sensor signal rationality code. The SCR catalyst itself may be functioning normally; the fault is the downstream sensor not reporting accurately.
How do I confirm the sensor is truly stuck and not responding to a very clean exhaust?
Briefly command a slightly fuel-rich condition or turn off DEF dosing via a scan tool function test. If the downstream sensor does not rise in response to either change, it is likely stuck.
Disabling P2E4C in software
RaceTune can permanently disable P2E4C — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECUs with a P2E4C disable in our catalogue
Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.
- Bosch MD1CP002 verified 1 software version
- Bosch MD1CS001 verified 1 software version
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P2E4C in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
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