Catalytic converter

**jamccor** · 02-26-2019

So my 2008 Pontiac grand Prix has had the CEL on for a few months now. First it was the crankshaft sensor. Then it was the upstream O2 sensor. After replacing both of those the CEL came back on. Had advanced auto scan it and the same codes keeps coming up. P0171 Bank 1 system too lean and P0420 Catalyst System Efficiency below threshold Bank 1. Since inspection is coming up and I cant pass it with the CEL, any ideas what it could be before I spend $100 for a shop to diagnose it? I know one of the symptoms with a bad CC is rotten egg smell which I do not smell. What other things can I look for to self diagnose this? How would I know if its a downstream O2 sensor? I have also heard stories that even if you replace the CC, the code doesn't always go away unless its a GM built CC.

**Scottydoggs** · 02-26-2019

id look for the vac leak first to fix the lean code. it might be triggering the cat code. OBD-II DTC: P0171 System Too Lean (Bank 1) as per the link, could be the maf going bad too.

**jamccor** · 02-26-2019

Scotty
So you think that it may be related to the MAF instead of CC?
What other signs of a bad CC are there? I've been looking online and have seen some wild prices for a new CC but the job itself looks very easy as long as it's not welded.

**Scottydoggs** · 02-26-2019

maf is in the list of things that can be bad in that link. the car na? or supercharged? how many miles? high miles and na the cat could be clogged. its what they do.

the lean code is usually a vac leak.

if you have a t app you can check the live data on both o2's and your fuel trims to see whats going on with it better.

**jamccor** · 02-26-2019

The car is not super charged. It's got 190,000 miles. What's a t app?

**Scottydoggs** · 02-26-2019

torque app. shows live data scans codes, clears codes. you need a droid device, and a elm 327 obd2 bluetooth. the elm goess for like 10 bucks or so. the app is 5 bucks.

there is a free app i found works with the elm as well. ChevroSys Scan Free (OBD2 & ELM327 for Chevrolet) - Apps on Google Play this one does abs srs and p codes. ive used it, damn thing works well.

**Scottydoggs** · 02-26-2019

and at 190,000 the cat is likely shot.

**mcdavidhome** · 10-29-2019

Hey Scotty I have a scan tool that can show me the live data but I just dont know what data is good or bad. I can see the waveforms and the fuel trim percentages but what numbers are considered good or bad how can you tell?