Probably been covered but I cannot find a definitive answer. I own a 2004 3.5l wagon and it has decided to run in limp mode. Are there numerous reasons for this to happen? Or just a few. I am thinking I need to sell the car cheaply to a handy man if the stories I am reading on here are the norm.People spending $3K and $4k to keep their cars on the road. Generally, the 2004 Magna's are only worth around that sort of money.
Meanwhile I would like a few pointers, if that's possible.

Check if the heater core is leaking. Coolant drops onto a relay and it goes into limp mode.

Happened to me two years after the heater core was fixed. The coolant had corroded the relay. Easy fix.

New heater core fitted could cost around $500, or DIY but dash has to be dismantled.

Yes, theres a few reasons for limp mode. As per above, if you can smell coolant in the cabin then that could be the reason.
But you really need to get the computer error codes read. Theres a guide here somewhere on how to do that with a multimeter, or you can get a dealership to read it. That will give a clue as to whats gone wrong, rather than guessing. Also, I *assume* that you already checked the ATF level.