I chased P0173 ‘Rear Adaptive Max Limit ‘ for the better part of a year on my car. First, those are frustrating codes. The order of areas I would check are: 1) Fuel pressure measured at the schrader valve at the manifold. 2) Air leaks at the intake manifold where it meets the cylinders. 3) Exhaust leaks at the exhaust manifold. 4) C02 Sensors.
If you are getting both codes I have a hard time believing both intake manifold gaskets went south at the same time - unless you recently installed a new air filter.... As I understand those codes...... somehow either 10% more or less fuel or 10% more or less air is being found (or requested and not found) by your ECM. If that makes sense. Search P0170 and P0173 - there are a few threads on this issue.
I have a G56 Exhaust on my car and installing (or making) a free-flowing exhaust alone, or even with an aftermarket free flowing air filter, will not cause those codes.
In chasing P0173 I replaced one not so great fuel pump that would not hold fuel pressure, switched the C02 sensors in the exhaust to see if that changed the code (it did not), replaced both intake manifold gaskets, and finally installed Emil's Fuel System Upgrade kit. No more code with Emil's system. Cannot say enough good things about Emil and his system that delivers fuel to both fuel rails at the same time the way S&S designed the fuel system to operate. And a real fuel filter BEFORE the injectors and an actual fuel pressure regulator with a gauge and ability to adjust fuel pressure is a bonus.
Good luck. Search the codes themselves on here for more info.
