Insighta into cause of p0420 error
 

I have a 2003 xtreme 2wd 2door blazer with 107k miles. I bought it last week. The old owner had the catalytic converter replaced in April. Once i got it, the p0420 error appeared. I took it to the people who replaced it, and they said i had a faulty o2 sensor.. So i went ahead and replaced all the o2 sensors thinking this would solve the issue.

But the error has returned.. Same one... Is there a default reset period or is there something else wrong?

could be the cat they installed. po420s fairly simple. either cat or sensor.

Isn't an exhaust leak also a possible cause of a P0420?

i dont think so. that would trip the upstream sensor,saying system lean. (oxygen false reading from the leak)

I replaced all the spark plugs and wires.. The light has not come back since then.. thanks!

When the PCM runs the monitor for the converter, (it is not continuously monitored) the fuel mixture goes from one extreme to the other, then back to normal. The PCM watches the time it takes to register the change at the downstream oxygen sensor. If the time is too short, P0420 sets. A hollowed out or missing converter will set P0420 every time. If there's an exhaust leak upstream of the downstream sensor, the exhaust is diluted and the test results are skewed. If there are no exhaust leaks, and all of the oxygen sensors are performing correctly, the P0420 is probably correct, "catalyst efficiency below threshold". Possibly meaning a cheap quality converter was installed, or not big enough capacity for the engine.

So i lied.. The light is back. I took it to a mechanic to test the cat, he said the cat was fine.. I recently replaced the fuel filter, and air filter but no changes yet... I am new when it comes to this kind of stuff... If they did put a cheap and insufficient cat on, how can i check it?

A different mechanic said that it might be a plug in the injection system or a map sensor.

what ive learned from being a mechanic the last few years, all these magnaflows, walkers, and every other cheepo universal cat only last about a year or so. ive been here about 2 years and i would say about 2 times a month one comes back that i installed when i first started working here with a reoccurring p0420 code.

its kinda funny to see how much my welding has improved from when i first started here. the problem is no one wants to spend the 600 or whatever crazy amount the dealer wants for a cat so they just throw on the 100 universals and you just have to replace them ever year.

and for as many cats as ive replaced, 50 or so for the 420 code all of them were fixed by replacing the cat. ive never seen one burn out as fast as yours did though, maybe you have another issue with the truck, dumping fuel or oil down the exhaust causing it to go bad, or maybe just the inside of it broke ive seen that before where the honey comb just exploded inside

I replaced mine with a Magnaflow direct fit, seems like it was $125. Been on there just over 3 years and 40K miles. Knock on wood, no problems.... yet. I agree on the el cheapo universal jobbies, they're junk. The direct fit converters bolt right on, and are a breeze to replace, no welding involved.

There has to be a reason that the P0420 set. Follow the flow chart step by step and you'll find the problem. If everything checks ok, the converter capacity is probably under sized for the engine.

EDIT: If there were a problem with the MAP sensor, it would have set a DTC for the MAP sensor.

Thanks guys for your suggestions.. I will look into the flow chart to see what is going on. I found out that the cat was actually replaced in November of 2012, so it might have gone bad... Is there a flow chart on the forum somewhere? I think i saw one, i will go back and check.

I'd get a scan tool with a data display and see what the o2 sensors are saying. Or find someone how can do it for you. But ya, I agree with above that these codes are simple. If its not the downstream sensor then it's the cat, sorry but it's wasted money to just replace all the sensors.

It's not as simple as a faulty sensor or converter. Any one or more of several things can set a P0420. Here is the flow chart: https://blazerforum.com/forum/diagno...plained-19820/

i had my exhaust leak fixed today (stolen cat) and on my way home my SES light came on (which i kinda expected) i stopped off at autozone and had them scan it to confirm my suspicions and yepp P0420. i was under the influence it needed to be welded on could you explain how to bolt it on? i gotta put a cat on to clear the light. do you have a link to the Magnaflow cat Captain Hook? i have a sawzall so i can chop off the straight pipe and maybe fix this myself

sky_blazer, On 2000, the direct fit converter is part of the "Y" pipe. There are universal ones available, and they really should be welded on. If you use clamps it will most likely leak, which could set the P0420 ;)

i gotcha now i looked on ebay and saw direct bolt on have flanges to bolt on, i looked under my blazer and whoever cut the cat off cut the flanges off as well which makes my job harder looks like im going universal, but i cant weld

Take it to a muffler shop, they can weld it in for you. Universal converters are rated to handle up to a certain cubic inch engine displacement. Make sure you get one that will handle more than 262 cubic inches or you'll likely see the P0420 again.

ok thanks i plan on ordering a universal from either autozone or ebay, ill have to get another downstream o2 sensor to while im at it. BTW until i can order this stuff will i experience poor mileage? or are those the ones in front of the catalytic convertor? also what kind do i need? i keep seeing certain state emissions for them

Downstream oxygen sensors do not affect engine performance, or mileage. They are there to monitor the converter, that's it. AC Delco and Denso sensors have the fastest reaction time, and Denso sensors are cheaper. If the thieves took the oxygen sensor bung, (the thing that the sensor screws into) you'll need a new one of those too. It gets welded into the pipe behind the converter.

i still have the bung so i also have the flange the bolts to the muffler too would you still suggest a universal type and have it welded in? i might be able to buy a direct bolt in unit, chop off the front flange, get an exhaust coupler, clamp the front on, and bolt the back on. ill probably experience exhaust leaks though right?

The system needs to be sealed, (no leaks) from the engine to the aft converter sensor. If there are any leaks, the P0420 could be set.

i got the light to turn off for now..i did the spark plug non fouler trick and it seems there are no trouble codes. i still gonna put a catalytic convertor on, but until i can save enough to do it i dont have to live with the annoying SES light on