I'm helping my 16 year old son work on his second car (first is a 1970's Bradley GT)

He bought a 1997 960 wagon for $600 knowing it would need some work. when we test drove it it ran a little rough but nothing that scared me off, but we got the price reduced due to the up arrow and lights flashing on the ESW switch. we did some research and were fixing and tinkering as he came up with money. we replaced the pnp switch and symptoms didn't change. we checked and cleaned all electrical connections on drivers side of tranny. no change. Swapped in a new used TCM and it all went away. the car idles smooth as can be. next he changed spark plugs and saved up for new tires. when we went to pull it out of the garage 3 days later runs really rich and up arrow and flashing ESW back on. suggestions on next step. I'd like him to stay engages and excited about this project and need help maybe pinpointing rather than just throwing out fixes as he can afford them,

thanks in advance