2000 Yukon Shudder And Stall

[wcat1]

My 2000 Yukon XL starts great but begins to shudder and then stalls as the engine idles down. SES ligth is on. Tried to read codes but code reader says the unit is not connected. I tried all possible combinations of sequence to connect, power on, and ignition switch on with no change. Code reader works fine on other vehicles.

 

Could these two issues be related? Any quick tips on what would allow a repeated good start but not remain running?

 

Thanks for any advice you can provide.