P27CA

NOx Sensor Circuit Range/Performance (Bank 1 Sensor 2)

P27CA is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Range/Performance (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.

Code
P27CA
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P27CA means

P27CA is stored when the engine control module (ECM) detects that the signal from the downstream NOx sensor (Bank 1, Sensor 2) is outside the expected operating range or is not tracking predicted values during active SCR system operation. The downstream NOx sensor is positioned after the selective catalytic reduction (SCR) catalyst and measures the residual NOx in the exhaust to evaluate SCR conversion efficiency.

The ECM continuously compares the downstream NOx sensor reading against the modeled exhaust NOx concentration based on engine load, DEF dosing rate, and catalyst temperature. If the measured value deviates from the predicted window by more than the calibrated threshold for a sustained period, this fault is logged. Common triggers include sensor contamination from DEF crystallization, wiring harness faults, or a degraded SCR catalyst that produces abnormal NOx readings.

Driving with this fault active may result in incorrect DEF dosing since the closed-loop SCR feedback is compromised. Prolonged operation can lead to increased tailpipe NOx emissions and may escalate to a derate condition if the SCR system is deemed non-functional by the ECM.

Common causes

Most-frequently reported root causes when P27CA is logged.

  • 1
    Downstream NOx sensor contaminated by urea (DEF) crystallization deposits.
  • 2
    Open or short circuit in the NOx sensor signal or heater wiring harness.
  • 3
    Corroded or damaged NOx sensor connector pins.
  • 4
    Failed or degraded NOx sensor producing out-of-range readings.
  • 5
    SCR catalyst damaged or depleted, causing abnormal NOx conversion output.
  • 6
    DEF quality issue leading to incorrect SCR reaction chemistry.
  • 7
    ECM software fault or calibration mismatch for NOx sensor plausibility.

Symptoms drivers notice

Malfunction indicator lamp (MIL) illuminated.
SCR system warning light or DEF system alert on the dashboard.
Possible increase in tailpipe NOx emissions.
Incorrect DEF consumption rate (too high or too low).
Possible engine power derate on some vehicles after extended fault presence.

How to diagnose P27CA

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all stored DTCs and freeze frame data before clearing anything.
  2. 2
    Check live NOx sensor data for Bank 1 Sensor 2 and compare against Bank 1 Sensor 1 under various load conditions.
  3. 3
    Inspect the NOx sensor wiring harness and connector for damage, corrosion, or DEF crystallization.
  4. 4
    Verify sensor heater circuit resistance and supply voltage at the connector.
  5. 5
    Inspect the SCR catalyst inlet and outlet for urea deposits or physical damage.
  6. 6
    Test DEF quality using a refractometer to confirm correct urea concentration.
  7. 7
    Replace the downstream NOx sensor only after wiring and DEF system checks confirm the sensor itself is faulty.

Vehicles where we've handled P27CA

Platforms in our catalogue with confirmed P27CA coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can I drive with P27CA active?

Short distances are generally possible, but extended driving risks escalating to a derate or SCR system lockout. Address the fault promptly.

Does P27CA always mean the NOx sensor is bad?

Not necessarily. Wiring faults, DEF contamination, and SCR catalyst degradation can all cause this code without the sensor itself being defective.

Will clearing the code make it go away permanently?

Only if the underlying cause is resolved. The code will return once the ECM re-evaluates the NOx sensor plausibility during a drive cycle.

How does the ECM know the sensor reading is wrong?

The ECM uses an internal NOx model based on engine parameters and DEF dosing to predict expected downstream NOx levels and flags deviations beyond a calibrated threshold.

Disabling P27CA in software

RaceTune can permanently disable P27CA — 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.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P27CA 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 EDC17C50 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.

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