Hey Scotty I have a 99 Lexus es300 was running fine, got the oil changed we noticed that it would hesitate on speed.. sometimes a small kick back. We changed out the spark plugs and even the coilpacks for good measure. Ran fine for two weeks started the decrease in power and sometimes even SHAKING at a stop, or lower speeds now.. Havent driven it since then. Took it to a muffler shop since a mechanic said it could be a exhaust leak. Let me not forget we scanned the car and it gave me a code for the Knock Sensor and P0420, P1133-P1153. Muffler shop told me that we're going to have to change BOTH manifolds and get a new Y pipe. Now when i look up the parts i see one for the manifold converter for the front I see another one for "front left".. would these be the two parts the muffler shop was telling me about? He just told me go on amazon look up "exhaust manifolds" and a "Y pipe" .. but some sites make it so confusing when they state the Y pipe for the front..or mid.. Rock auto only gives me front Y pipe so im assuimg thats the one needed.. Im just a bit lost at the converter exhaust. told me if the repairs are gonna cost more than what i paid for it to invest somewhere else.. but if its an "easy" fix why not jus fixit..
Before you get into that manifold R&R-- did you check your O2 Sensors and/or Air Fuel Ratio Sensor(s)? Replace any that are faulty; have the OBDII professionally cleared (re-set), then run OBDII diagnostics again.