Hello Ben I will just ask some of the questions again ....
If you go into the data blocks now after the boost pressure replacement what is your ignition on reading on the sensor that you say has some variation on ?
Is the replacement boost sensor you have a got a good make ? Not ting tong ?
You are using vcds ??
On the first page is advanced measurements highlighted ? So you can go into the readings like this ......
**This is not a A8 its just for an example** ......113 ambient pressure 115 actual boost pressure .
I just want to be sure we are reading the correct values for actual boost and atmospheric pressures .
Regards Ecms ......yes they have the pain in the arse security plates over the top of them normally with the snap head bolts ......no easy way round this I'm afraid , I either grind a deep slot in the snap head bolt and use a big flat screwdriver to undo ...or if you cannot get at the bolts just grind through the plate which ever way you can .
Before you do that.... with ignition on you need to check at the boost sensor ( disconnected )plug that you have the ' strong' 5 volts supply and a ' strong ' earth
Use a volt Meter and a normal bulb test light to test these wires like I explained in the earlier posts ......there's good reasons behind this method ....and for not using led Type test lights or just using a volt meter .
There's a reason why you want to test the 5 volts and Earth at the boost sensor plug also .....see ....you could test at ecm end and they could be present ......doesn't mean that they are present at the boost sensor plug though .....if the loom has corrosion or chaffed through wiring the volts or earth pathway could be compromised from Ecm to loom plug .
So let's say the 5volt supply and earth is good , the other two wires ... boost signal and air temp signal measure the voltages off them wires ...just as an example let's say them wires are 2 volts at the sensor end ....they should be 2 volts at ecm end also .
Hopefully some of our other members could pull a wiring diagram off the Vag Ekra site
My gut feeling is that this will be a poor earth at the sensor ....but carry out the tests ......... You have heard me say this before ......test ...rule out move on ....test not guess .
Ben if you need more help i will pm my number to you .