SES Light P0420 - Need help diagnosing / replacement

mubai

Original poster
Member
Jan 5, 2012
321
Had this code come up today. I have 185K miles on the truck and never replaced the O2 sensor(s) or CAT.

How should I go about diagnosing the issue? I saw Chickenhawk's post about checking the pressure (I would have to buy a pressure gauge, no big deal). I feel like it does struggle a bit on acceleration, but I can't be certain. I drove a 100 miles today back from work and I calculated the MPGs. The DIC indicated 20.5 MPG, actual miles/fuel consumed showed 20.8MPG, so I feel like I can trust the MPG's are in that neighborhood. This has decreased from the 22 MPGs I used to get a couple years ago, but again could be the route, weather, quality of fuel, tire tread...

So what steps should I take in diagnosing this?

It's not 4:20 or anything about "trees." :popcorn: It's an OBDII code P0420. :rotfl:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
mubai said:
How should I go about diagnosing the issue?
I gotta ask if you found my posts on this exact code on trailvoy? Always start with a search, targeted if possible like this:

Google for "P0420 site:trailvoy.com" or "P0420 site:gmtnation.com"

Get a scan tool that can read real time data and look at the O2 sensor outputs.

The P0420 code is generally for a cat that isn't working well, or bad sensors. The upstream O2 sensor should have varying levels that change once or twice a second. The cat does its job and the downstream O2 sensor should have a flat, non-changing output. In my case with an intermittent P0420, the downstream OP2 sensor had an output that varied JUST LIKE the upstream one, implying the cat wasn't clogged, but it was not there. Looking closer, I had put a big rock ding in the side of the cat a few thousand offroad miles ago, and the ceramic cat innards had broken apart and departed through the muffler. When we cut open the cat's housing, 2/3 of it wasn't even there! :eek:

At 100K, did you replace the upstream O2 sensor along with your plugs?
 

mubai

Original poster
Member
Jan 5, 2012
321
the roadie said:
I gotta ask if you found my posts on this exact code on trailvoy? Always start with a search, targeted if possible like this: Google for "P0420 site:trailvoy.com" or "P0420 site:gmtnation.com"
I found many posts but I didn't see one that provide diagnostic steps.

the roadie said:
Get a scan tool that can read real time data and look at the O2 sensor outputs.
I don't have one. Not sure which one to get. Also, not sure if I want to spend $150 on one. Pepboys charges $50/30mins for diagnostic services. I was hoping not to shell out any money for diagnosis, but if that tool is the only way, then I'll probably have to suck it up.

the roadie said:
At 100K, did you replace the upstream O2 sensor along with your plugs?
Never replaced my 02 sensor. Plugs were done around 100K. I guess my other option is to replace the O2 sensors and see if that fixes the problem. I know its "throwing parts at the problem," but considering my mileage, it may be necessary?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
mubai said:
I found many posts but I didn't see one that provide diagnostic steps.
I assume everyone has the set of PDF manuals available from a member here. If Googling wasn't fruitful. "Low Cat Efficiency" means the cat is NOT cleaning up the exhaust, and the output sensor waveform is the same as the input sensor waveform and they should not be. It can only be the sensors or the cat, and a bad sensor would usually set of other codes than just P0420.
Not sure which one to get.
One that does more than read codes, but also displays real time sensor data for the O2 sensors. We can't tell what you can buy used on Craigslist, so you have to get familiar with Googling for model numbers you see in your local ads.
Also, not sure if I want to spend $150 on one.
If you have an android phone, get Torque and a bluetooth dongle. Much less.
Pepboys charges $50/30mins for diagnostic services.
Fair price, if it's being run by somebody who knows what real time O2 sensor data should look like.
I was hoping not to shell out any money for diagnosis,
Why? A good diagnosis is a thing of value. You'll hate to see the cost of a cat, which a P0420 usually means you have to replace. Sorry.
but if that tool is the only way, then I'll probably have to suck it up.
It's the best way to avoid shotgunning.
Never replaced my 02 sensor.
The longer they're in without being changed, the harder they are to change when they eventually die.
Plugs were done around 100K. I guess my other option is to replace the O2 sensors and see if that fixes the problem.
It won't, 95% chance it's the cat.
I know its "throwing parts at the problem," but considering my mileage, it may be necessary?
Shotgunning is never NECESSARY, it's always an economic tradeoff between getting an accurate diagnosis versus the time and $$ to do it right. A good scan tool is an investment to make sure you're not Pep Boys bitch forever. Get a group of friends to share one. Go to a meet where somebody else has one. Check Craigslist for ads by people who own such a tool and do it cheaper than Pep Boys.
 

mubai

Original poster
Member
Jan 5, 2012
321
Cool. Thanks, I'll see if I can find a good one. Might try to get one of those you can hook up to your laptop or I might checkout some apps for the iPhone.

OK, thanks again.
 

MAY03LT

Member
Nov 18, 2011
3,422
Delmarva
Check yo email! I'd love to get a love plot of a failing converter on video.:cool:

edit: live plot, not love plot, thanks roadie
 

MAY03LT

Member
Nov 18, 2011
3,422
Delmarva
That's when Oda Mae Brown reads the lines on your hands and tells you when you're gonna score. With her.:undecided:
 

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