P27EF

SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

P27EF is a generic OBD-II powertrain diagnostic trouble code: SCR NOx Catalyst Efficiency Below Threshold (Bank 1). 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
P27EF
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
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What P27EF means

P27EF is stored when the ECM calculates that the SCR (selective catalytic reduction) catalyst on Bank 1 is not converting NOx at the minimum required efficiency. The ECM derives catalyst efficiency by comparing upstream NOx sensor readings (before the SCR catalyst) against downstream NOx sensor readings (after the catalyst) under steady-state conditions where DEF dosing is active and the catalyst is at operating temperature.

Normal SCR catalyst efficiency typically exceeds 80-90% under moderate load. When the calculated efficiency falls below the manufacturer-defined minimum threshold and the condition persists through a validation drive cycle, P27EF is logged. Root causes include physical degradation of the catalyst substrate, DEF quality issues, incorrect DEF dosing due to related sensor or doser faults, and catalyst poisoning from lubricant additives or fuel contamination.

This fault is emissions-relevant and directly indicates elevated real-world NOx output. Depending on calibration, the ECM may impose an engine power derate after the fault has been confirmed over multiple drive cycles. DEF system health, including the doser, DEF quality, and both NOx sensors, must be verified before concluding the catalyst itself is at fault.

Common causes

Most-frequently reported root causes when P27EF is logged.

  • 1
    SCR catalyst substrate degraded or thermally damaged from overheating.
  • 2
    Catalyst poisoned by phosphorus, sulfur, or other contaminants from engine oil or fuel.
  • 3
    Incorrect DEF quality or diluted urea reducing ammonia availability for NOx reduction.
  • 4
    DEF dosing injector clogged or failed, reducing DEF delivery to the catalyst.
  • 5
    Upstream or downstream NOx sensor fault providing incorrect efficiency calculation data.
  • 6
    Exhaust system leak between upstream NOx sensor and catalyst diluting the NOx measurement.
  • 7
    Extended idling or low catalyst temperature preventing adequate SCR reaction.

Symptoms drivers notice

MIL illuminated with possible SCR or emissions system warning.
Failed or borderline emissions inspection result.
Engine power derate on vehicles with escalating derate calibration.
Normal DEF consumption but no improvement in NOx reduction.
Possible co-existing NOx sensor or DEF doser DTCs.

How to diagnose P27EF

A typical diagnostic flow when this code is present.

  1. 1
    Read all stored DTCs and record freeze frame; check for companion NOx sensor or DEF doser codes first.
  2. 2
    Verify DEF quality using a refractometer to confirm 32.5% urea concentration.
  3. 3
    Check DEF dosing injector operation using scan tool actuator tests.
  4. 4
    Review live upstream and downstream NOx sensor data across a range of engine loads to calculate real-time efficiency.
  5. 5
    Inspect for exhaust leaks between the upstream NOx sensor and the SCR catalyst inlet.
  6. 6
    Confirm catalyst inlet temperature reaches the SCR activation window during the drive cycle.
  7. 7
    Replace or clean the SCR catalyst only after all sensor, doser, and DEF quality checks are satisfactory.

Vehicles where we've handled P27EF

Platforms in our catalogue with confirmed P27EF coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Can bad DEF cause P27EF without a bad catalyst?

Yes. Diluted or contaminated DEF reduces ammonia production, directly lowering SCR efficiency below the threshold even with a healthy catalyst.

How many drive cycles does it take for P27EF to set?

The ECM typically requires the efficiency to be below threshold for one or more completed drive cycles with the catalyst at operating temperature before confirming the fault.

Will replacing the SCR catalyst fix it permanently?

Only if the catalyst is genuinely depleted. Leaving doser or DEF quality issues unresolved will degrade a new catalyst in the same way.

Is P27EF a federal emissions failure?

Yes, SCR efficiency faults are classified as emissions-relevant, and the MIL-on condition is reportable during official OBD inspections.

Disabling P27EF in software

RaceTune can permanently disable P27EF — 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 P27EF 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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