1998 Buick Skylark Code P0420

I have a 1998 Buick Skylark with a 3.1 litre engine. At approximately 115K miles I took it to a local mechanic & had a complete tuneup IE: plugs, wires, hoses & serpentine belt. After having the tuneup
I drove about 400 miles & the check engine light came on with Code P0420 I took it back to the mechanic & he said that the lower intake gasket was bad & he replaced it. Again I drove about 400 miles & the Light came on again with same P0420 Code. This time he replaced the oxygen sensors & then the light came back on again within about the same driving distance same code. This time the Catalytic Converter started making noise so I took it to a muffler shop & the Converter was completely burned out on the inside. I had them replace the converter & I then drove about 2000 miles & now the light is back on again. I then had a scan done (Autozone) and Code P0420 code came up. As you can see I have already spent several hundred dollars & the vehicle still has problems & of course will not pass inspection. Can someone possibly help me with what the problem might be. As soon as the light came on I have not driven it again because I don't want to damage the new converter. I had never experienced this problem prior to the tuneup. I guess my next step will be to have it checked by a garage that has more sofisticated test equipment. Any help & advice anyone could give me to get this problem resolved would be most graciously appreciated. Thanks Robert W. Joyner
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Bobby12333 wrote:

Post back with voltages for all the O2 sensors.
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It sounds like you have an over rich condition. You need to find a mechanic with the capability to analyze the problem with an OBD2 scanner. It is possible that the plugs or other parts installed for the tune up are the wrong parts for your engine....

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Thanks very much for your input. I have been told about a reputable garage in my area & I am taking the car to them today. Thanks again, Robert
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Hey Guy's I took my 98 Buick Skylark to a reputable mechanic & they kept it for two days checked everything with their sosfiscated equipment & everything is within specs & ECM tests were good. They turned off the MIL, road tested it several miles with scanner attached & light did not come back on everything looked normal so now they are scratching their heads also. I had them check the plugs & wiring & the plugs are AC OEM & are the correct ones for my car according to the owners manual & all parts that were replaced on the 115K tuneup IE: Plugs, Plug wires Fuel filter, Serpentine belt & Radiator hoses were purchased from a Carquest Parts Store & that is the Parts Store that most of the independant garages use in my neck of the woods. Some do use NAPA. Mechanic at this Garage gets all of his parts from Carquest. Thankfully the Garage owner did not charge me anything since they could not fix the problem. I feel sure that the MIL will come back on again because it has haunted me so many times in the past few months. The car runs perfectly gets 30+ miles to the gallon but it will not pass inspection if the code P0420 sets again in my location. I have pretty much exausted most of my alternatives & this is really frustrating. I know this is a long message & I apologize but I wanted to try to explain everything I could so someone out there might be able to help me. If anyone has any suggestions they would be greatly appreciated. If I had plenty of money I would probably light up a match & put this car out of it's misery & give me some satisfaction. Thanks in advance for anyone who can help!! Robert Joyner
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Bobby12333 wrote:

Post back with voltages for all the O2 sensors.
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I just wanted to post back & tell you guy's that The MIL has come back on again after driving 223 miles since I last took it to a garage that performed all tests & turned off the MIL since they could not find a problem. One person had mentioned to post back with 02 sensor voltage readings. The Mechanic did not remember the exact voltages from my first visit but did tell me they were all within specs. Anyway I have taken it back to the same garage & told them to keep it as long as needed to see if they can pinpoint the problem since I have another vehicle I can drive. May sound a little crazy but after they turn light off I can fill the car & it will always get to between 3/4 & 1/2 tank of gas & then the light will come back on. By the way I have also replaced gas tank cap. I have relayed all this info to the garage owner. The owner of the garage is going to test drive as long as needed with scanner attached to see if he can come up with any answers. He has graciously agreed not to charge me anything at all unless he can fix the problem. This garage is highly recommended in my area as being fair & honest in these type of situations. I know this is a long letter but I figured you guy's would be interested. I will post back & let you know what the mechanic comes up with as soon as I get the car back. Man this has been one mind boggling problem. Robert Joyner
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Bobby12333 wrote:

Hi Robert...
Not a mechanic, just an old retired electrical guy who loves cars... so take my thoughts for what they're worth :)
Anyone considered the possibility that there may be a crack or pinhole in the fuel tank dip tube? Perhaps at 3/4 of a tank the fuel sloshing could expose an air leak intermittently, and at a half tank do it even more often.
Yeah, I know, this should set a lean fuel code if anything, but.... suppose the pump sucks air for a tiny time. Computer says gee that's real lean. Richens it up. Then the fuel sloshes, sucks pure fuel again, and it's waaaay over rich. Possible?
Just thinking...
Ken
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Ken, Thanks for the info. I did relay the situation to the mechanic about the problem happening after filling up then the light coming back on between 3/4 & 1/2 tank. With the situation with my car I guess anything is possible. The mechanic will keep & test drive the car as long as necessary to see if he can come up with a solution. I have read & read on OBDII Forums & have not seen where anyone else has encountered this exact problem. This is an interesting problem & I intend to post back & let everyone know if the mechanic comes up with a solution so it might help someone else in the future. Thanks to everyone for your replys. Robert Joyner
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Hey Guy's It's been quite a while since I wrote about the emission problem I was having for so long with my 1998 Buick. The mechanic scratched his head for quite a while, all readings were good & he finally just decided to have the converter replaced (for the second time). Well of all things it turns out that the converter has an arrow to indicate the direction of flow & the muffler shop that installed the converter put it in backwards. My mechanic has agreed to go to the muffler shop that installed it with me to take up the issue with them & hopefully get a refund from them for the first converter. I can only hope that this information will keep someone else from going through this frustrating situation that has taken 7 months & much$$$ to finally figure out. I just had the vehicle inspected today & it passed with flying colors. Thank heaven I believe this horror is finally over. Bobby Joyner
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Post back with voltages for all the O2 sensors.
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