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Hello and thank you for you question,Can you tell me what fault codes you are actually getting?
You may very well be right with checking the exhaust back pressure because the H2 has a long history of restricting Catalytic converters so I would sure do that first and see if you feel a power lever return. Try loosing the exhaust up on both banks at the exhaust manifolds till you have a healthy leak and then take it out for a drive. If there restricted which is very likely then you should feel some increase in power.
If after checking the back pressure and you find no difference the the other thing you should try which can cause the P0300 code to set is run the Crankshaft Variation Learn Procedure. See the steps in the link below for this procedure. Copy and paste the link to your browser to open and view them.https://dl.dropboxusercontent.com/u/104923897/Crankshaft%20Variation%20Learn%20Procedure%20Nov%2019.pdf
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