safety mode

About three weeks ago my 2000 beetle wouldn't start. Took it to the shop and they found a bad battery cable. They replaced the cable then they said the oil light started flashing and found that the antifreeze is mixing with the oil which typically means bad gaskets. I've decided to sell the car...

I have just replaced a busted radiator on a 2000 Beetle 2.0L. The repair went smoothly... so I thought. About 20min into a test drive, I got the check engine light and a loss of power. It seems like it is in some type of safety mode. I have not run any diagnostics to find out any codes. Any ideas?