2004 Dodge Caravan Rough idle

Yes, the engine light is on. It's been on since we purchased the van (used) in 2009. We've had diagnostics run on it time & time before and nothing came up on the "machine". This last time it was run it came up with the code notifying us that and EO2 sensor needed to be replaced. Times are tough and we hadn't had the $ to replace it. That's about all that's been done to it since the roughness started. Which was about 3 wks ago. Could this be the problem? Because the sensor wasn't changed things got worse? A tune-up was done (according to the shop mechanice at the used dealership we purchased the van from) 6 mts before we purchased the van. Could it need a tune-up? How would we go about checking the fuel pressure? This is our 1st time purchasing a used vehicle, so we are not at all sure what to do or what we should of done. Thanks again for replying. Your help is greatly appreciated! Have an awesome day!

Thank you so much for the link! I will get my husband to do the check as soon as possible. He's a corrections officer and works noon till 11 pm, so as soon as he's able to do a fuel check, I will definately let you know. Do you happen to know where the O2 sensor goes? My husband has no clue. We could change it asap. We are planning a trip out of town on the 28th and want to get this taken care of before then. Thanks so much for you help. I will get back to you as soon as possible.

P.S. We have the repair manual, the one Autozone sells.