1999 Sentra no power

[codecxbox]

Hi..just got a 99 Sentra std trans and it looks like motor was recently repaired..I installed a new clutch kit and general tuneup..current codes are P0136 and P0400..I already cleaned the TB , the 2 EGR valves and replaced related rubber hoses. Also cleaned the IACV and the MAF.
However I have very low power overall..I hardly can get to 45 mph at WOT at 4th gear.On hills I have to constantly go from 2 to 3rd just to get pickup.My 95 1.5 civic with a bad head gasket is a whole lot better.
I already ruled out everything engine related..all is fine and its right at the timing mark at 8 degrees at idle..I checked the ecu with an OBD2 and Touchscan for windows. no chain slap or noise either.Gas consumption is outstanding..no smoke at the pipe even with a heavy load
Question is, if I have a P0136 code which means my rear O2 sensor is not functional, could it be a clogged cat and not show a P0420 just becuase of the non working O2 sensor?
Thanks beforehand..This is pretty frustrating, I need this car..nice gas saver..

[Sideways s12]

Yeah I would change the O2 and see if it gives you the other code. Then go from there is your cat welded in or bolted in?

[codecxbox]

Tks Sideways..turns out the the cat was gutted..so I was looking for that pesky Po400 code and removed the 2 egr valves...I noticed there was hardly any vacuum from the intake egr port tube..So I removed that and sure enough the intake passage was clogged with carbon all the way..I cleaned that with carb cleaner and a soft wire and put back everything..Now I can up press the egr valve and it stalls...before cleaning it woulndt stall..been 2 days and not having a code yet..3rd shift is a lot better now..also seafoamed the intake..lets see what happens..ill keep you updated..