Service engine soon!!!!!

[D Mc Gee]

My service engine soon light has been coming on recently. I would like to access the codes, but my Haynes manual says not to plug in my jumper wire unless the engine is off. Once the engine is off the codes are lost, right?? How do I pull the codes?

Thanks,
Diesel newbie
Doug Mc Gee
92 Chevy 1500 6.2 auto

[ropinfool]

The codes should not be lost until either the condition is repaired or goes away and after a certain number of cycles or the batteries are disconnected. So plug her in and see what you get. John