Code P0420

[SSEI173]

Hello, I have had an on and off problem with my check engine light coming up with the low catalyst efficiency code P0420 in Threshold Bank 1.

The first time it came on my mechanic replaced the Catalytic Converter. I thought that it would be smooth sailing from there but it turned back on, I thought it might just be a glitch or something or maybe the mechanic reset it wrong somehow because everything ran and sounded the same with no change in performance or anything, so I reset the code.

I just replaced my 4 year old spark plugs with NGK TR55 V-Powers as I am trying to get my MPG and performance back, I have slowly gone from about 17 or 18 MPG down to 12.5 MPG, really bad, even for the 30 degree winter mornings, with short stop and go commutes daily.

After driving the car around with the new spark plugs in everything sounded alright, I drove it for about 5 minutes checked under the hood and all was well except I heard a slight "ticking" sound, almost like knocking but not exactly. It wasn't too loud and everything else checked out so I thought nothing of it.

I then hopped on the freeway and opened it up to a 75 MPH cruise, it ran fine, then as I approached my exit and went back into the city my check engine light P0420 code came back on. I have heard that sometimes the cars computer wont run tests unless its at a certain speed for a minute or longer, and this was the first time in a while being on the freeway at that speed for a while, so that may be why its coming on now.

Just wanted to know your thoughts over here, will I need to replace the O2 sensor? Is it possible my terrible MPG is due to a bad O2 sensor? Also, I made sure all spark plugs were gapped and installed correctly, but is it possible that I may have caused a connection to come loose or bumped something while changing the plugs causing the code to come on?

Also, just a side note I've got the problem where the D.I.C. displays low oil level at startup every time even though it is full of oil and has been kept up on oil changed, will I need a new oil level sensor on the crankcase? Estimated cost?

Thanks in advanced for the help, you guys are always helpful here.

[Archon]

Some of the after market cats have been known to cause that code to be set. Some have had the computer set to ignore that code, or buy a rear O2 simulator. That O2 sensor will not affect your gas mileage. If you don't know how old the front sensor is, you may want to replace that. Also, try cleaning the MAF sensor and check to make sure your air filter is in good shape.

You likely will need to change the oil level sensor, bur first check the connection to be sure it is clean and tight. Check the parts stores for prices, or send a private message to harofreak00 for a good used one.

[Mechtech]

How long did the new cat last before P0420? Did the shop put on the cat and not check the car for a cause of the failure. Were the old plugs black and fouled?

[SSEI173]

Sorry about the really late response, I've been out of town for awhile.

I replaced the Spark Plugs and tested the wires, wires are good and the old spark plugs weren't fouled just had a lot of natural wear on them for 5 or 6 years worth of use. I replaced the plugs and my MPG went from 12.5 MPG to 14.5 MPG.

I checked the connections you guys mentioned and found no problems. I still have the P0420 code, when they replaced the catalytic converter at their shop they told me that the reset the code and it should be fine, however it is still coming on. Not sure about the age of the Oxygen Sensors but they are expensive to replace. I will do the MAF sensor because I know in the cars 129,000 miles it hasn't been changed.

Are there any suggestions on how I can diagnose the problem before I go drop $100 on an O2 sensor? If that is what is causing the problem.
Thanks for all of the help on these forums so far guys, I'm learning a lot about my car as I work on it, something I've wanted to do for a long time, but I never had a car to work on.