I have a 2004 Subaru Forester that won't rev past 4500 at first I thought it had a clogged cat since it was throwing and P0420 code. Replaced cat and O2 sensors and cleared the codes. The codes are now gone, the cur runs smoother, but still it won't go past 4500 rpms. The car boggs down and starts losing speed , when I let go of the gas and press it again it shifts up to the next gear. What could it be? Oil levels are fine in the car.
Do you really need the engine to go beyond 4500 RPM? What's the redline on that thing, maybe 5000? Does the engine have a rev limiter, particularly if it has an automatic transmission?
Seeing as how the car is 20 years old almost anything could be wrong with it. You might want to analyze live data for problems. Also see the troubleshooting section of the site FAQ.
@chucktobias the problem I'm having is going up hills, it would struggle going up hill because the engine starts bogging down at 4000-4500 rpms and it starts losing speed which is not something you want when going up hill. Car redlines at 6500.
Gearing in that thing must be pretty wild, I've never had to run an engine that fast to get up a hill. Check the usual suspects (filters, spark plugs, fuel pressure, exhaust back pressure) and dig deeper if not any of those simple things.