How much information is retained in the on-board diagnotics?

I've been getting repeated non-starts and some dies-while-driving
during the recent spell of hot,humid weather up here in Toronto.

Chatting with the guys in Suter's Garage while making an appointment
for my wife's S40 service, we think that it's probably that dad-blamed
cam-sensor that causes simialr problems and has been talked about
elsewhere.

So, I want to drop by the shop and have them do a reading for what's
going on. MY question is how long will the information be retained
(tiem and engine starts?)

And I may want to change the sensor myself and save the time in the
shop. Just where is this sensor located and how hard is it to replace?

Thanks in advance!

Bruce
Firm believer in 4x60 air conditioning until you start sticking to the
seat and then it's MAX AC with FAN=4.....