Hey all, I have a 05 blazer 4x4. It had the spider injector replaced in January because it had a hard miss and that fixed it but with that it blew apart the exhaust right after the muffler a bit and made a gap there. Ever since I have had P0420 (only code) and notice low power under load (mostly when going up hills in 4x4) Post cat o2 sensor fluctuates like the pre cat does. Fuel pressure at 60 and passes leak down test. I’m hesitating on pulling a pre cat o2 and driving it because I don’t want to start melting stuff. Thoughts on how I should go about it to confirm the cat is bad before I replace?

 

This. Also you can check response time when changing mixture ratio for an appropriate delay (this shows capacity). Best to plot things. Just reading changing numbers is a futile exercise. I'll see if I can find a particular old thread where I responded on this quite a while ago.

Here it is https://blazerforum.com/forum/2nd-ge...0-99042/page4/

See in particular posts 33 and 43

 

Thanks! I just read that thread, I will try that trick with the brake booster later today hopefully. My post cat sensor is definitely not nice and level like the “good cat” example either. I saw the spark plugs actually fixed it for the guy on the thread, I might pull one of my plugs too and have a look as I haven’t looked at any of them since buying this beauty a year ago

 

If you want to rule out a mixture problem like the spark plug guy start by looking at your fuel trims. Also that OP probably needed to do both, replace the dead cat and then fix the exhaust gas problem caused by the plugs that killed the cat. Thats why you want to fix any fuel delivery and ignition problems before replacing your cat so that you don't end up back in the same place.

Here is the process that Les is referring to with a detailed video:




George

 

Thanks for the information guys, I have looked into this a little more and pretty much concluded my cat is done. I think I will replace the plugs and wires when I replace the cat just for preventative maintenance even though my fuel trims are looking good. Looking at new cats online I see magnaflow wants $700 for a direct fit? Does anyone recommend anything else?

 

Wanted to update this. Never replaced the cat. It was idling quite bad and would have an attitude starting every once in awhile (priming wouldn’t help and also wasn’t an evap issue). Up until this I only had that cat code. One day I leave to go for work and it would NOT start unless I had my foot heavy on the gas pedal and I would need to keep the gas held once started. It was idling at like 300rpm, misfiring, did not feel good and within about a minute I had a low idle code and p0300. Replaced the iac valve and it is running MUCH MUCH better the idle is a night and day difference from even before when it would still run. No problems starting it at all anymore and best of all the p0420 cat code has stayed away. I noticed the p0420 code would always set at idle aswell so I am hopeful I have fixed this problem!

 

Oh well, you can lead a horse to water....

 

So your post cat O2 sensor was tracking your upstream sensors and after the IAC fix the downstream sensor is now reading roughly flat between .4 and .7 V?

George

 

Haha I love that Les. I was thinking dumb that I could kill 2 birds with one stone with the new iac. The code came back today, I guess I was never driving it in a state where it would cycle the test until today? Anyway I guess the cat is truly bad and I will need to cut off my arm and leg to afford this direct fit

 

Never got around to hooking the scan tool up and checking that out while the cat is warmed up yet. I presume since the code came back it’s the exact same thing as it was before though.