I have a '99 626 with a 4 cylinder engine and AT. It drives perfectly when cold but after a few minutes the transmission just slips like it's in neutral.
It had a p0421 code and CEL was on. I replaced both O2 sensors, PCV, sparkplugs and wires. Now no more CEL or codes. Voltage is 12.5v engine off and 14v running. Trans fluid looks good and is at correct level. It has a trans cooler. No vacuum leaks I can find.
Any suggestions?
TIA