Had my FD for about a week now, bought it with a bad coolant sensor which was causing the car to start poorly and trip the ce light....since then I've replaced this and it was fine for a day or two, and now its come on again! The car is running fine not hot or anything or any obvious signs of something being wrong, but the oil pressure gauge malfunctions and randomly works/dosent. I bought it like this and I havent checked if its the actual guage or the sending unit, could this be tripping the ce light?

 

Start by checking the codes:

http://www.scuderiaciriani.com/rx7/engine_codes.html

That will tell you what is causing the Check Engine Light (CEL).

 

The CEL may be stored or you may have multiple codes, which are easy to miss. You could try disconnecting the negative cable and push on the brake for 30 seconds to try to reset the ECU. Also, the oil pressure guage's erratic readings, especially at low rpm, is not uncommon. The sender is located near the base of the oil filter pedestal. Try cleaning the connection. I doubt it's the guage, but it may still be a bad sender.