ENG DIAG Switch Do Anything?

[RustyTools]

2006 HR Endeavor with ISL400. The dash has a Carling-type switch labelled ENG DIAG, with momentary-up or momentary-down positions. I am getting an intermittent Check-Engine-Light when I start the coach, but pushing that switch (in either direction) doesn't do anything.

Two questions come to mind...
1) Is the switch even connected?
2) Does the code have to be active when I push the switch, or can the switch/ECM read/display stored codes too?

[Mike Canter]

Normally that switch has two functions. One is to make the CEL flash so you can read the trouble codes. This is normally down with the key turned on and the motor not running. The other function is to bypass a STOP engine light and keep from shutting down the motor.

[RustyTools]

(Hi again, Mike)

Ok, I just talked to Cummins and they said the CEL light (and switch) will only blink the code when the code is active. I was thinking it would flash any stored codes too, but if the code isn't active at the time I don't get a light.

I had an issue like that years ago in a car. Don't remember now what the fault was, but it was only intermittent and the ECM didn't store the code and so the dealer could never find it. As luck would have it, I was literally one street over from the dealer one day when it lit. Buzzed right into the service area and started jumping up and down, yelling: "Get The Reader! Get The Reader!". Bet they thought I was nuts.