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1998 Plymouth Voyager

I have a 1998 Plymouth Voyager. A couple of months ago I noticed my van began misfiring, rough idle, change in gas mileage, etc. I was also smelling gas fumes in the car. I had a private mechanic check it out. He pulled the code p140. He said both O2 sensors needed replaced. He did this, van ran great for about a week, then check engine light came back on. Van began running badly again. Had the spark plugs and wires replaced, ran great for a few days, then same issues again. Took my van into a shop. The mechanic said code P140 came up. I told him I had just had both o2 sensors replaced. Mechanic said they were the wrong specs and I needed to have them replaced…again. Paid $450, van ran great for another week, then same exact issues. Check engine light came on. Van runs better at higher speeds. Very rough idle, almost feels like it is going to die. When accelerating off of a start I have to put the gas pedal to the floor. The misfiring is almost unbearable.

Before I take it in again, I am wondering what the plausible problem is. Could this be a bad fuel injector? Is it normal for the O2 sensor code to come up when that is not ultimately the root cause?

Any input would be greatly appreciated.

This would be your rear O2 sensor, but I doubt that the sensor itself is bad. since replacing it seems to work for a week, I would look into the vehicle side of the connector for that sensor. There is the possibility that one of the pins is contaminated or corroded, or that water has gotten into the connection.

Something may also be FOULING an O2 sensor. Make sure the cat converter is operating to spec and that the car isn’t running SUPER RICH.

Also, is the car burning any oil as though the engine is about worn out?