Thread: 01 GTP stalling randomly not starting

Ive done extensive searches trying to diagnose this problem and almost thought I had it taken care of.
Heres the story.
Bought an 01 GTP a couple months ago. Started driving it daily about a month ago. When I bought the car the guy said it had a stalling problem but he thought they fixed it with a new fuel filter. Obviously thats not the case.
My girlfriends father is the service manager at a GM dealer. He brought it into the shop and said it was the mass airflow sensor. So we replaced it.
After that it still had been stalling rarely but starting right back up. Every few days or so. Also I noticed off the line at WOT it pings something mean around 3500-4000 rpms. So I avoid making it do that at all costs.
Then it started to stall everyday and lit up the SES. Then a few times a day and not starting again for ~5 minutes. Checked it with a basic scan tool and got a 0420 low efficiency bank 1. Then even cold it wouldnt start unless you cleared the code. Now it seems that the 0420 code is a problem many people spend lots of money trying to fix. A lot of the answers i found said clogged cat or bad rear O2. Girlfriends father seems to think that bad rear O2 wouldnt cause it to stall and not start. Im also unsure if a clogged cat would make it stall randomly and sometimes not start again.
Like how could it be not so clogged it can run most of the time but clog up to a point where it chokes it out other times?
SO he thought it was the front O2. He brought home that super GM scan tech 2 or something tool home and we replace the front O2. After that everything seemed golden. He checked everything with the tool and it seemed fine. And for 2 days after the change it only stalled once and started back up. Yesterday it stalled in traffic and wouldnt start again for about 5 minutes. Then this morning same thing and this time SES light back on. I dont currently have the scan tool but when i do ill pull the code. Its probably 0420 again. And it still pings.
I use premium fuel only from places like mobil or shell. No ****ty cheapo places.
This car has no mods other than my stereo setup. It had a K&N filter but he said that might have been what killed the MAF but IDK so theres a paper one there now. I checked all the vac lines and some are cracked a little but I taped the **** out of them until i get new ones.

I have this heavy feeling that the pinging is directly related to this stalling issue. Or its been running wrong for a long time and the cat is getting more and more clogged. I also thought maybe bad cam position sensor setting the timing off but there hasnt been any codes for that.

Any help would be great. If anyone needs more info I can give it other than the current code. The scan tool may be back tonight if its in the truck but if not ill have it monday night.

The line that connects under the snout was cracked and taped by someone before me. Actually the t connector is cracked. I retaped it. It will be replaced very soon.
As for crank position sensor I thought if that was going my tach would drop to zero while running which it doesn't. A clogged cat would cause a 0420 code. Ill do a pressure test. Would a clogged cat cause pinging?

I may just unbolt the cat and go for a rip to check it. Im pretty sure im also lacking power. This is my first l67 vehicle so I don't know what they are supposed to feel like when operating properly. I used to own a V8 Taurus sho with 5 less hp and it was much quicker than my gtp which isn't supposed to be the case.

147k miles of new england driving. Should make for a fun Saturday.

So I see the only place to unbolt the cat is right after the o2 sensor. Not like it would make life easier but I was hoping it was just bolt on at each end. And I don't have the o2 sensor socket anymore. So time to dig for an open end 22 mm.
Hollowing it is an option as long as I get the rear o2 simulator to trick the computer during inspection. Visual inspection isn't an issue since my girls father also does our inspections and he doesn't care as long as the inspection computer doesn't. Honestly if I only had a welder id weld a straight pipe in. That u bend is awful.

So I see the only place to unbolt the cat is right after the o2 sensor. Not like it would make life easier but I was hoping it was just bolt on at each end. And I don't have the o2 sensor socket anymore. So time to dig for an open end 22 mm.
Hollowing it is an option as long as I get the rear o2 simulator to trick the computer during inspection. Visual inspection isn't an issue since my girls father also does our inspections and he doesn't care as long as the inspection computer doesn't. Honestly if I only had a welder id weld a straight pipe in. That u bend is awful.

Just an update.
I got to thinking with the help of you guys. The rear o2 was reading lean. Around 4k rpm it would ping badly. Post o2 reads lean so it adds fuel. Too much to burn and exhaust. I wasnt able to pull the new front o2 to test the cat. I ordered a new one anyways. Came in on friday. Had to only cut the head off one post cat o2. Was able to turn the rest of it with vice grips. Got the new cat in without a post o2. Again I didnt have a 7/8 wrench. Luckily it came with a blank plug installed so i didnt have a leak. Its like night and day. Car is a god damn beast compared to what it was. No pinging whatsoever. Pulls strong up until 6500 when it shifts. Even shifts better while just cruising around. RPM's are lower when just cruising. Like i said i didnt have a 7/8 tho a 22mm would have been closer. It was still stalling randomly and much more often than before without a rear o2. Went out this morn and bought a 7/8 and got the o2 installed but havent been able to take it on a rip.
Even if it still stalls I can now pinpoint better what the problem is. The old cat was choking the **** out of the motor and it should be easier to pinpoint whats wrong without that issue.
I think what clogged the cat was a bad maf. It was bad when i bought it but by the time it was replaced it was too late for the cat. If I turn the old cat in my hands it sounds like a rain stick.
Im just happy the power is there. Much quicker than my SHO.
I didnt delete the u bend. Dont have the proper cutting equipment or a welder. But i will say the u bend in the replacement isnt nearly as flat and choked as the stock.
I ended up with an eastern cat assembly. "High flow" ceramic. Not legal in CA.
Eventually I will do an exhaust. I just wanted to get the thing operating normally before mods.

Thanks guys.