SES light On Solid

Hi - I have a '97 SC1 with a newer SES issue

One time in the past at approx. 65,000Km the owner told the shop it came on and went off few days later - dealership was still able to get a code without the SES being lit & told customer (previous owner) it was a bad sensor(rear)and they replaced at N/C as a courtesy (weird since the rear is costly)

anyway, I now have turned approx. 226,000KM with it coming lit solid but disappeared a week later

so are the codes stored and can they be pulled and/or cleared with the obd2 while the light is no longer lit?

some mechanics told me to wait b4 asking them to hook it up and others are saying it shouldnt matter. Thing is the Parts Source shop will hook up (the OBD2) at N/C but only when the SES is still lit.

the first time it happened to me was a month or so ago and it went away on its own - now it has come and gone (within a week again) b4 having the chance to go there while it was still lit. Lol

I want to ask them to pull the code for me without arguing about the light not being lit, a bit tricky if they are set in their ways or can I encourage them to humour me? Or is it wait for the light to come back again? Otherwise the car is well maintained & runs perfect. Im leaning towards a gas cap or rear 02 sensor again but would like the codes to be sure. Would appreciate any advice? Thanks

ps, a little skeptical with donating at this point but will not hesitate down the road - thanks

richard