98 Jimmy SES Codes - Please Help!

[98JimmySLS]

Hi to all!! I must say, I have used this site for years and have gotten countless amounts of information from it. You guys have saved me thousands of dollars. Now I need y'alls help again and can't seem to find a thread for it.

A little background information:

I have a 1998 GMC Jimmy SLS 2WD with 157,561 miles on it. I have had this vehicle for 6 years with no major problems. The catalytic converter was replaced 2 years ago, water pump replaced 9 months ago. About 6 months ago, I replaced the spark plugs, wires, distributor and gas cap.

In January, this vehicle was in an accident due to an icy bridge. It hit the guard rail on the front passenger side, spun around and hit the passenger rear also. I fixed the front end damage myself, since it was mostly cosmetic except for the radiator and fan. I also took this time to change the oil and transmission fluid and filter. The vehicle idles fine. It has been driven short distances, not over 50 mph, except for one 3 mile trip down the freeway at 70 mph.

Now for the problems:

1. The vehicle is sluggish in drive and reverse at startup. It can be felt shifting into these gears, but when the accelerator is pressed, it's slow to move. It drives fine after it's warm.

2. When at freeway speed, I noticed the rpms fluctuating from 3000 to 5000 and seemed to have a difficult time holding speed. When I pressed the accelerator further, it downshifted and rpms almost redlined.

I took the vehicle to the local AutoZone because the SES light was on. The codes are as follows:

P0420
Catalyst efficiency low-bank 1

P0452
Fuel tank pressure sensor condition

P0453
Fuel tank pressure sensor condition

I need to know what these things mean and how to go about fixing them. This vehicle was an everyday driver that spent an hour and a half on the freeway round trip. Any help you guys can give me would be greatly appreciated!!

[MT-2500]

What engine?
What kind of spark plugs did you feed it?
You have more problems than them codes.
And may have more codes.
Any flashing check engine lights?
I would start with a good tune uo and fuel pressure check.
What is your fuel pressure readings?

Any other codes or engine missfires?

Repair flow charts for codes can be found in a good repair manual or a all dat DIY online repair info sub.
http://www.alldata.com/products/diy/index.html