Mercedes-AMG A45: a new valve, the same fault, and why we stopped swapping parts.

What came in

A 2013 Mercedes-AMG A45 with fault codes stored across several control units. Our first full scan showed faults in a long list of modules.

The first job with a scan like that is to find out which faults are current. We cleared everything and scanned again. Some came straight back. The ones that mattered were in the all-wheel-drive control unit: a fault on its pressure regulating valve, and a message that the data it was getting from the ESP was implausible.

Testing the valve

The pressure regulating valve is a solenoid. The obvious first question is whether the solenoid itself works.

We took it off and tested it on the bench on 12 V. It clicked. We cleared the faults and scanned again, and they returned.

The next day we noticed something more useful. The faults set as soon as the car moved.

A new valve

We fitted a new solenoid. The fault came back exactly as before.

This time we watched more closely. The fault returned the moment the gearbox was put into Drive or Reverse, without the car even moving.

What that tells us

A brand-new part throwing the identical fault is one of the most useful results a diagnosis can produce. It makes the original valve an unlikely cause.

The timing matters too. The fault appears when a gear is selected, before the car moves, so it does not need any load from driving to set. We read that as pointing at the electrical side: the wiring between the valve and the all-wheel-drive control unit, and the unit's own output, rather than anything mechanical in the drivetrain. That is a line of enquiry, not a finding yet.

Where it got to

As of mid-July the fault was still present with the new valve fitted. The next step is measurement, not parts: checking the valve circuit and the control unit's supply and output, so the decision on what to replace rests on a reading rather than a guess.

What to watch for

• A part that works on the bench can still fail in the car. Clicking on 12 V proves the coil is not open circuit. It does not prove the circuit around it is right.
• New part, same fault: stop and measure. If a replacement behaves exactly like the original, the next replacement probably will too.
• Note exactly when a fault sets. "When it moves" and "the moment I select Drive" sound similar but point in different directions.
• Clear and rescan before believing a long fault list. Old, stored faults hide the one that is actually current.

If your car has a fault that keeps coming back after a part has been changed, our diagnostics page explains how we approach it.