Error code PO 420

[Bazza]

Hi Everyone:

2002 SSEI

A while back I replaced my CAT due to it rattling. On a long trip shortly after I thought we might have come across some bad gas at a small country station, and the check engine light came on.

Code PO 420 came up.

I replaced the plugs, plug wires and checked coils. Code PO 420 returned in a short while, but car appeared to be running ok.

I replaced the before CAT O2 sensor which cleared the problem up for a while, but after a bit of spirited driving, the code came back.

I am thinking about replacing the after CAT O2 sensor.

Anything else I might be missing here?

[00Beast]

What brand cat are you running?

[Bazza]

I have no idea. It was installed by the local muffler shop. A jober perhaps.

(I was suspicious here also)

[00Beast]

Hm. How much did they charge you? An OE style cat would be well over $200, an aftermarket high flow would be less than $100. I have a feeling it's a high flow unit that is not doing enough catalyzing and your PCM is in a tizzy over it.

[Bazza]

The bill was about $350 bucks for the install. It was Midas that did the installation and they usually purchase parts through NAPA here.

The thing is that after the before cat O-2 sensor change, the check engine light would come on very shortly after I cleared it. After the O2 change, the check engine light did not come on again for months. (I didn;t use the car much over the winter however).

If anything I would say the new converter is more restrictive than the OEM. (Could just be my imagination though)

[Bazza]

Well I finally have the issue solved.

Turns out it was a dirty MAF sensor, which a local shop caught with their scanner. The signal it was sending was way out of range. (I had no way of checking this with my equipment at home)

The cleaning has eliminated the hard shifting, along with the error code. I did try cleaning the MAF with brake cleaner which did not work previously. My local guy tells me that only official MAF cleaner will remove hydrocarbons.

Through all this I was not able to generate a faulty MAF code.

[2002BonnevilleSE]

Another way to clean the MAF is by using rubbing alcohol and a Q tip. VERY gentle rub the small wire with the Q tip dipped in the rubbing alcohol. VERY being the key word.
BTW...I used a dremel and a small bit to take that PITA security thing out of the torx bolt. Now a standard T 20 fits fine.