My first question would be what kind of code reader the service center uses. some smaller shops have code readers that only display active codes. dealers and larger service centers generally have more sophisticated readers that will also read previously active codes. difference being a couple thousand dollars in cost sometimes. When you say in for service, does that imply an oil change? If so, my first thought would be a lower viscosity oil was used to refill. How many miles does your 2004 jeep have on it? Vehicles naturally pump less oil at lower RPMs and as they get older with more miles, the oil pump gets to the point that it can not pump enough oil at idle to maintain pressure. even a different brand of oil filter is enough to cause this problem in older engines. do you often look at your oil pressure at a stop sign or just now that the "check gauges" light came on? this may have been a gradual drop without you noticing until now. It could also be a simple thing as oil pressure sensor going bad. hope it is a simple fix and good luck.