2001 ES - Many problems...
 

Hey all. I have an 01 ES (4cyl) with 101k miles.

I have a number of issues and I was hoping to get some advice or see if anyone else has had these same problems and possibly figured out the solutions.

My check engine light never seems to go off. About 1 year ago I had these codes: P0421, P0335, P0116 and P0421pd. I reset my check engine light about 2 months ago and now I get a P0421 and P0421pd, they seem to be the fault code every time I reset accompanied by others. This time around the lucky partner was P0300 and so I replaced all my sparks with new NGKs and reset. Now I have the 2 P0421's, but this time with a P1500 code. I googled the code and I get: Alternator FR Terminal circuit. So I assume this means my alternator is on the crapper and ready to be flushed at any moment. I had a friend clean the ground connects of corrosion and battery terminals, we then reset the check engine light, needless to say a whopping 10 miles went by and now its back on. I will head to autozone to get a free alternator test and see what that tells me. I noticed my dash lights on the freeway would go dim and then light up and then dim, etc.

I intermitantly experience an issue where my car won't start. When I turn the key I get power, but the car won't turn. There isn't any clicking, just nothing, but after a few tries (cometimes after 15 minutes even...) it turns on. So I wonder if the alternator code is correct and could be related to this issue or if I have just yet another seperate issue on my hands.

I have had the following repairs since I have owned the car:
full tune up (at 68k)
coil replaced (filled with oil somehow)
New Battery (2 years ago)
Timing belt + water pump and pretty much all belts replaced at this same time (done at 75k)
Radiator replaced (8 months ago)
Intake Manifold (busted twice due to Tunex sucking at replacing my timing belt, failed to resync belt so my engine was shaking to dooms day until we figured out what was wrong)
AC electrical line repair
never missed a beat with oil changes (i use mobile full syn) and all other fluid flushings.

I have a good friend who owns a master muffler and tells me that the 0421 code referring to the Cat, is commonly a side effect due to some other component in the car that needs to be fixed, but could be causing the 0421 fault code.
I also experience my oil light flashing quickly when I am stopped or at a low rpm and after the car has been running for a few minutes. The car also shakes when stopped, not a terrible shake, but enough to make the glove box rattle.
I dont seem to be suffering any power loss, I accelerate just as well as I did the day I bought my car (bought it at 62k).

I just need to get things in order so I can re-register my car soon. I also don't want to throw too much money at the car, I want to try and be as educated and accurate as I can be in repairing. I love my car and I try to take good care of it. Thanks for any help and I am happy to offer any info that I did not include. I am no mechanic by any means. Thanks for your time.