PO402 code catalytic converter

I've used this site a lot to figure out some problems with my truck so thought I would share this
story, hope it helps someone.


CEL came on a few weeks after smogging my truck in califiornia. guys at auto store pulled up the PO420 and told me that my cat was probably done. Thought it was strange because the truck passed with good numbers even though the cat is original and the truck 20 years old.
The engine burns clean too, no fuel smell in exhaust plugs looked great no other symptoms that would fry a cat. So I looked under the truck at the exhaust system searching for air leaks, cracks in the manifold etc. Nothing. while I was doing this I had the engine idling for 10 minutes or so. turned off the engine, started to close up the garage then looked under the truck one more time and then I saw it.... Water dripping from the flange behind the cat and just before the rear 02 sensor. Hmmmm if water can get out then air can get in. Removed the bolts off the flange and looked at the gasket. Thing was toast. Picked up a new one put it on, reset the CEL and bang. No more CEL. Again a lot of the things that I tried came from this site. Thanks everyone.

 

Good catch

fresh air leaks are often to blame for cat codes

 

Bad gaskets cause this code to trip often.

One needs to make sure to get the correct gaskets I had parts stores get gaskets mixed up hanging them up.

Wrong gasket fails in no time yet most think I just replaced the gasket that can`t be the problem.

 

Yes true it took a while to find the correct gasket there are so many that claim to be the one. This is what the old one looked like. I am assuming it is original. Not bad for 20 years of living inside a muffler shaft...

 

Hm, I'm in a similar situation. Bought a 06 access cab with 6 speed MT, got 40 miles from the dealer and it threw a similar code for my CAT. Went through a bunch of hooplah with the dealer but long story short they put in a new CAT and new O2 sensors. Truck is still throwing the same code. Hopefully it will be something simple like this, just gotta get under there and look. Thanks for the read!

 

Also, remember that not all converters are created equal. I'm willing to bet that in an attempt to save money this dealer probably installed a cheap converter that does not have as much strength has the OEM one. OEM converters are more expensive because they have more precious materials built in (palladium, rhodium, etc) than aftermarket. Take a picture of the new converter while you're under there. Can you see a brand name on it? If its one of those cut and weld universal ones it may be part of the problem

 

The dealership was definitely a bunch of shysters so I wouldn't be surprised. I was able to get them to cover all the costs because they tried to scam me...

 

Lots of used car dealers will clear a check engine light and then hope somebody buys the vehicle before the diagnostic monitor gets a chance to run again. Not trying to be rude but this is very common practice. Did you consider a pre-sale inspection? Looks in your case that it took about 40 miles before the converter (or fresh air leak) coded again.

Advice for next time buying a used vehicle.. any $80 code reader should allow you to look at the "ready-ness" state of the monitors. You can see if codes were recently cleared, the amount of driving since codes were cleared, which monitors have been given enough time to run (passed / failed) ,etc. I check this stuff on every used car/pre-sale inspection that i do

 

Yeah, I learned the hard way.

 

You saved quite a bit cuz of your due diligence. Way to go !!