P25AC

Reductant Pressure Too Low During Regeneration

P25AC is a generic OBD-II powertrain diagnostic trouble code: Reductant Pressure Too Low During Regeneration. 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
P25AC
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
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What P25AC means

P25AC is set when the PCM or aftertreatment control module detects that diesel exhaust fluid (DEF) system pressure has fallen below the minimum threshold during an active SCR regeneration or dosing event. Adequate DEF pressure is essential for precise atomisation of urea into the exhaust stream upstream of the SCR catalyst.

Low pressure during regeneration can stem from a blocked or frozen DEF line, a worn or faulty DEF pump, a clogged DEF filter, or a leaking pressure line. If pressure is insufficient, the correct quantity of DEF cannot be injected, leading to incomplete NOx reduction and potential catalyst degradation over time.

The control module logs this fault after pressure readings during an active dosing or regen cycle fall below the calibrated minimum for a defined period. In addition to the MIL, some calibrations will enter a reduced-performance or de-rate mode to protect the aftertreatment system. Diagnosis should start with a visual inspection of the DEF tank level, lines, and connections before testing the pump and pressure sensor.

Common causes

Most-frequently reported root causes when P25AC is logged.

  • 1
    Faulty or worn DEF (reductant) pump unable to build adequate pressure.
  • 2
    Blocked or kinked DEF supply or return line.
  • 3
    Clogged DEF filter restricting flow.
  • 4
    DEF tank level too low for the pump to prime.
  • 5
    Frozen DEF lines or pump in cold-weather conditions.
  • 6
    Leaking DEF line fitting or injector seal causing pressure loss.
  • 7
    Faulty DEF pressure sensor providing incorrect low readings.

Symptoms drivers notice

MIL illuminated and possible SCR system warning on the instrument cluster.
Reduced engine power or torque de-rate active.
DEF system fault message displayed to the driver.
Possible increase in exhaust NOx emissions.
DEF pump audible operation may sound abnormal or absent during start-up.

How to diagnose P25AC

A typical diagnostic flow when this code is present.

  1. 1
    Record all active and pending DTCs with a scan tool before clearing anything.
  2. 2
    Check the DEF tank level and verify the fluid is genuine urea solution of correct concentration.
  3. 3
    Inspect DEF lines and connections for kinks, cracks, freezing, or leaks.
  4. 4
    Check the DEF filter service interval and replace if due.
  5. 5
    Monitor DEF pressure via live scan tool data during a cold start pump self-test.
  6. 6
    Test DEF pump output pressure against the manufacturer specification.
  7. 7
    Inspect the DEF pressure sensor and wiring for damage; replace the sensor if readings are implausible with known-good pressure.

Vehicles where we've handled P25AC

Platforms in our catalogue with confirmed P25AC coverage.

MB GLE350 30D

Related powertrain codes

Frequently asked questions

Can cold weather cause P25AC?

Yes. DEF freezes at approximately -11 degrees Celsius. Most systems have a heater circuit, but a heater fault can cause low pressure in cold conditions.

Will topping up the DEF tank clear P25AC?

If a low tank level caused pump cavitation, refilling may resolve the root cause, but the DTC must still be cleared with a scan tool.

How urgent is P25AC?

It is moderately urgent. Continued operation without repair can result in progressive de-rate cycles and potential SCR catalyst damage from incorrect dosing.

Is the DEF pump the most common cause?

Pump wear is common on high-mileage vehicles, but always verify fluid level, filter condition, and line integrity before condemning the pump.

Disabling P25AC in software

RaceTune can permanently disable P25AC — 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 P25AC 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 EDC17CP57 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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