P252A

Reductant Injection Air Supply Control Circuit Low

P252A is a generic OBD-II powertrain diagnostic trouble code: Reductant Injection Air Supply Control Circuit Low. 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
P252A
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P252A means

P252A is stored when the ECM/PCM detects a low or open voltage condition in the control circuit for the reductant injection air supply system. In air-assisted SCR dosing architectures, a dedicated air pump pressurises the reductant delivery line; this code indicates the control signal for that pump has dropped below the expected threshold.

Common causes include a short to ground in the control wiring, a blown fuse in the circuit, or a failed output driver in the reductant dosing module. An open circuit between the ECM and the pump can also pull the signal line low through internal pull-down resistors.

When this fault is active the SCR system is unable to inject reductant, resulting in elevated NOx emissions. Extended operation in this state may trigger progressive engine derating strategies intended to ensure compliance with emissions regulations, ultimately resulting in a significant power reduction.

Common causes

Most-frequently reported root causes when P252A is logged.

  • 1
    Short to ground in the reductant air supply control wiring.
  • 2
    Open circuit or broken wire between the ECM and the air supply actuator.
  • 3
    Blown fuse in the reductant air pump supply circuit.
  • 4
    Failed output driver in the reductant control module producing a low-side short.
  • 5
    Corroded or backed-out terminal in the air pump connector.
  • 6
    Failed reductant air supply pump with an internal short to ground.

Symptoms drivers notice

MIL illuminated with P252A stored.
Reductant dosing system inoperative.
NOx output above regulatory limits detected by downstream sensor.
Reductant or SCR system fault warning on the dashboard.
Potential engine torque derate after repeated drive cycles without correction.

How to diagnose P252A

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and freeze frame data with a scan tool before any repairs.
  2. 2
    Check the fuse for the reductant air supply circuit and replace if blown.
  3. 3
    Inspect the wiring harness from the ECM/dosing module to the air pump for shorts to ground or open circuits.
  4. 4
    With the circuit disconnected, measure continuity between the control wire and chassis ground to confirm or rule out a short.
  5. 5
    Check connector terminals for corrosion, damage, or poor seating.
  6. 6
    Measure the resistance of the air pump motor winding and compare to specification.
  7. 7
    If wiring and pump are confirmed good, suspect the reductant control module output driver.

Related powertrain codes

Frequently asked questions

What is the difference between P252A and P2527?

P252A indicates a low voltage or open-circuit condition in the air supply control circuit, while P2527 indicates a high voltage or short-to-voltage condition.

Can a blown fuse cause P252A?

Yes. A blown fuse in the pump supply circuit will remove power and can result in the control signal reading low.

Does the reductant tank level affect P252A?

No. P252A is specifically about the electrical control circuit for the air supply, not the reductant fluid level.

Will the engine derate immediately?

Typically not immediately; most systems allow a number of drive cycles before initiating a derate, but the SCR system will be inactive from the first occurrence.

Disabling P252A in software

RaceTune can permanently disable P252A — 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 P252A 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 EDC17C56 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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