Fault P1251

This code is common when a larger exhaust has been fitted or other modifications have been carried out, however it can occur on completely stock vehicles.

P1251 is most often defined as... "Stepper motor intermittent"

Where in fact this code is an over boost code, the stepper motor gets the blame as its in control of the VNT and therefore the boost control.

How the system works:

In the lower load levels, the ECU does not ‘close loop boost control’. Basically, the ECU has a predetermined ‘VNT position’ table based on the loads. This dictates ‘roughly’ the boost levels, and given the low loads, close enough is seen as good enough. When the load levels come up, the ECU gets into its closed loop, and therefore will do whatever is required to maintain its ‘target boost levels’.

Common causes of P1251:

Basically, anything that causes the engine to produce more boost than it should. some examples are:

High flow exhaust added

This is one of the most common 4wd upgrades done and therefore one of the higher contributors to these issues.

Piggyback chip

As a piggyback chip is designed to ‘trick’ the ecu, there ends up being more fuel than the ECU knows about and thus creates more exhaust gasses which in turn ‘over spool’ the turbo.

*This is NOT the case for remap!

Higher flowing injectors / nozzles

For the same reason above!

Failed Stepper motor

Due to the failing stepper motor, it holds the VNT in the incorrect position and causes the issue.

Failed Turbo

Least commonly of all, the actual VNT assembly itself has failed or become physically seized within the turbo.

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