Honestly, I'm not a mechanic. Just going on personal experience of owning a 95 Blazer. I had a good mechanic put me in that direction years ago when I first had the problems. I've taken it off and cleaned it a couple of times but once I had the truck tuned up and running right, it hasn't been a problem since for a while now. And believe me, I've went the route of replacing everything but the right part...cpi injection, fuel pump, plugs, wires, and nothing made a difference until we pulled the egr off and literally found a chunk of coal holding it open part way. My buddy explained to me that it was supposed to be closed at idle and open when your on the gas. And btw, I never had to buy a new one, I just cleaned mine out real good and have been going strong...for the most part.

 

well I took the egr off and it looks pretty clean. What do I clean it with?

 

P0420 is not generic, it states there is an efficiency issue with the cat. You keep referring to a Bank 1 code, what code is that?

Cleaning O2 sensors is usually futile, you could wire brush the outer "screen" but the sensing part inside is unaffected.

 

I think the point is being missed. When I run the scanner it reads. CODE P0420 GENERIC. Its just an OBDII CODE. Catalyst System Efficiency Below Threshold (Bank 1) Thats the direct quote from my scanner and the dealers. That is where the Bank 1 comes from.

The cat was replaced in October of 2010. I do not think it is likely it is bad considering I have had this issue before the cat was even replaced and it is still doing the same thing.

It says to check
MIS
FUE
CCM
CAT
EVA
O2S
HTR
EGR

So it is not just a code linked directly to the CAT. There can be a number of things wrong or linked to this code. With the CAT already replaced now you see why I am checking all other items linked to this. The dealer said to clean the O2S and see if that helps because when I motor is running as inefficient as mine was they can get coated. I know how to test them and clean them. I asked how to clean the EGR. A wire wheel cant do anything for that and I don't want to spray something toxic in there and cause it to fail. Tomorrow I also plan to check the full exhaust for any leaks(as suggested on a number of sites) and check the CAT to make sure it in still intact.

After that I don't know since everything else has been ruled out and or replaced. This is not a new problem. Also I had a dye added to my coolant and then used a light to check for leaks and in the oil and there is nothing. My coolant level to perfect and has maintained it.

 

I used a piece of stainless steel welding rod as a poker to scratch and scrape at the inside as best I could. The first time I cleaned it, I didn't realize the whole inside was caked solid. If it is caked up, it will chip out with a little effort and then just blow it out real good afterward. And make sure the plunger or whatever opens when you push on it and closes when you stop pushing on it. Hope that makes sense.

 

Yes. That makes sense. It moves freely now but I'll scrape away and hope something comes out and blow some canned air in it. Thanks. I have a new O2S on its way. It will be here Thursday or so. I made some calls to the Chevy dealer and also a few other smaller shops and they say try replacing the plug in front of the cat because the one after is reading that there is no change in air quality after passing through the cat which typically means the one right before is failing. Makes sense to me and they all said the EXACT same thing. So I'll clean these parts and try the O2 when it gets here.

NOTE:
I bought this blazer in 2004. I still had the work sheet in the glove box from dealer when i first bought it. I bought it "as is no warranty"(mistake).
Well I remembered I had two O2's replaced when I first got it because it dies on me on the highway a few weeks after getting it. The code they ran was P0420 and they replaced two O2's and it fixed it. Go figure.

 

Ok, I mis-spoke, The reason I say Bank1 is not significant in this code is that there is only 1 CAT and both engine banks flow into one exhaust pipe and CAT. Both bank1 O2 and bank2 O2 monitored exhaust flow feed into the same CAT. That is why you need to read those sensors and other parameters while running and diagnose where the problem is. And yes an exhaust leak (before the O2 sensors) is an item to check for this code. Don't get hung up on the fact that the code states bank1, suspect either or both sides (banks).

 

I cleaned the EGR. Is was way dirtier then it looked at first. Also cleaned the MAF. I checked for exhaust leaks and everything looks great. The CAT looks perfect. I took the O2 out that is right before the CAT and realized this is one I replaced like 4yrs ago. It is a bosch. So maybe since it is not AC Delco it is not holding up or something. I cleaned it. The little fins where actually pretty clogged. I have an AC Delco O2 on its way and will just swap it around and see what happens. I am hoping I can find a shop that can or will read my O2's and be honest with me.

 

Okay so i installed the new AC Delco O2 sensor. Took it for a spin and CEL came back on. Still runs just like it did. But now only the HTR and EVA are blinking on the scanner. I already took the CAT(magnaflo after market direct fit less then a year old) and look inside it. Everything still seems fine. I made sure all bolts are still tight and everything fits. This problem has also been going on before I replaced the CAT so I do not think it is part of the problem. I cleaned the MAF with the cleaner from the auto parts stores. The code is still P0420. I'll pretty much try anything that could help so throw your ideas. I figure tomorrow i'll call around and see if anyone local can hook this thing up and monitor all the readings while its running. Thanks guys

 

Well I borrowed this $7,000 computer and monitored my cars reading. This scanner must be more sensitive though because it picked up 4 codes.

P0171 Fuel Too Lean Bank 1
P0174 Fuel Too Lean Bank 2
P0300 Random Cylinder Misfire
P0420 Catalyst efficiency threshold (Bank 1)

Any ideas?