Turbo charger boost sensor fault?

[jquattro]

This is from a 2002 A4 1.8TQM. I haven't seen this before. Can someone point me in the right direction?

Thanks!

[5ktq]

I'm gonna go out in a limb and guess there is a problem with the boost sensor, they do fail sometimes.

Do you have the code more specifically?
Eg implausible signal, signal too low, etc.

Basically ECU checks it when it starts up, if pressure isn't around ambient (which it should be when the car isn't running) it assumes it's faulty. Also if it goes to either rail during operation, etc.

(Or the wiring could be damaged, which would give you 0 or 5V which is out of range)

[jquattro]

Sorry, I should have included this information:

17964P1556098
(A)Circuit:control limit not reached

Thanks for the help!

[old guy]

I don't think it's a faulty boost pressure sensor. A P1556 isn't an electrical code. It indicates that you aren't reaching the specified boost. It. It is usually because of a mechanical problem. Something like a boost leak, a sticking N75 boost control valve or a problem with the DV. I would recommend starting with a boost leak test.

[5ktq]

Yeah I agree.

[jquattro]

Thanks for the help. I'll start with those items. I replace the N75 and Diverter Valve when we bought the car because we had to go in and replace the water pump; and I liked the idea of doing it when the radiator support was pulled away from the engine.

[jquattro]

OK, so I checked all of the lines from the N75 and they were good. Then checked the turbo waste gate bushing and it was good. Then did a leak test on the intake system and did not find any leaks. Then we turned the ignition on without the engine running and measured about 8.32 volts across the N75 valve electrical wires. So at this point we didn't have any fixes in site.

Then we clamped off the hose from the N75 to the waste gate and drove the car. Wow, that did it, the car ran great! So the N75 valve is bad, correct?

Thanks.

[A4SoftWalker]

^^^That is the correct conclusion.
By clamping the N75 line to your wastegate shut you took it out of the loop for boost control.

[jquattro]

Thank you!!