P0420

Hello,

I am wondering if anyone had this issue with check engine light code p0420.


I replaced the upstream o2 sensor... no change.

Thinking it might be my cat.

 

Same problem

 

Common code with several potential causes. I thought mine was the catalytic converter. Bought an aftermarket cat and while removing the original found the real cause: the exhaust gaskets upstream and downstream of the cat were not sealing, causing an exhaust leak.

 

p0420 can be triggered by many other things. Upstream exhaust leaks, vac hose leaks, non OE genuine Denso A/F sensor B1S1. Even crap gasoline over 10% Ethanol. Catalytic converter replacement only after testing with a heat gun.

 

You have an air/ fuel sensor B1S1
Never buy a cheap copy from fleaBay or sponsored seller on Amazon.

 

I got original denso ones

 

I will take a look at the gaskets then on Monday

 

Upstream of the 1st sensor is the place to look, also exhaust manifold. Have someone hold a rag on the tailpipe while engine is running while you check for the dreaded hissing of exhaust leak. Hope it's not a cracked exhaust manifold or bad gasket there. Good luck.

 

super easy way to find exhaust leaks: add some SeaFoam via the brake booster vacuum hose. All the leaks will become apparent in short time - just don't do this in a garage. I had a P0420 and found my original muffler rusted out on the very top where you couldn't see it. The rear 02 sensor mount was leaking due to rusty mount bolts that were ready to fall apart and the gaskets were virtually gone. I picked up a new mid pipe, muffler and exhaust gaskets. P0420 gone and never came back.

 

Catalytic converters don't go bad, they get killed by lean or rich fuel trim issues. If you didn't have any of these, your converters are likely to be fine

A very common cause for P0420 is exhaust leaks. Even pinhole leaks can trigger a P0420. I would check for this first

 

I found the exhaust leak

 

Is the exhaust leak between engine and O2 sensor?

 

Right before the muffler

 

Exhaust leaks downstream (after) the sensor won't cause PO420.

 

And you know the muffler from the catalytic converter right?

 

Yes

 

Why add it to the vacuum hose? Just curious

 

can knock on it too with a mallet and listen for rattle sounds, not exactly a 100% conclusive test but if you hear a lot of material rattling around inside the cat then that means it's deteriorating and on it's way out, if it's not already

 

No rattle