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SITUATION: 1998 Discovery 5.9L Cummins has intermittent check engine light. BACKGROUND: The issue can occur when idling (i.e., running engine at storage facility) but will come and go even when idling. The engine, at idle, does not rev or sputter, just has the light. The issue also occurs after highway travel and coming into town. When this happens, there is intermittent power loss while the light is illuminated such as stopping at a red light and then accelerating. Then, when the speed comes back up, it seems that the power loss is resolved and the check engine light goes off. There is no other gauge or indicator issues noted. ANALYSIS: I cannot find a connection for diagnostic trouble codes, but I assume there must be something somewhere. Under the dash, there are two rocker switches (image attached). Neither one seems to do anything, but I think depressing the "Engine Check" switch may have reset the check engine light. REQUEST: Where can I find the port for diagnostics? What is the function of the two rocker switches (image)? Advice on where to begin troubleshooting this issue. Thanks in advance!

2004 Winnebago Journey with Cat 3126. Long story. Started about 2 months ago coming from California to Mesa, AZ. About an half hour into my journey traveling with the cruise set at 65mph, the check engine comes on and I loose power. Stays on for just seconds and goes out and resume speed. All gauges check OK. About an half hour later, same thing happens. . Get to our Campground and make an appointment at the Frightliner Shop in Chandler, AZ. Arrive early and start work on it. Just observing, looks like that have it hook up to the computer for at least a half hour. Shop Foreman comes by and states they want to road test. Gone for at least a half hour. Foreman comes back and tells me that the Tech cannot find any problems. Give me a bill and sends me on my way. All is well for 100 mile trip to our next stop. Now a month latter, upon leaving the campground in Cottonwood, AZ the engine light comes on and goes into the limp mode. I get off the highway, park and turn off the engine. Wait a few minutes and try and start. Will not start. Wait some more. Will not start. I have it towed to a shop. The tech hooks it up to his computer and reads the codes. He then want me to try and start the Cat. It starts....He scratches his head, looks dumb founded and states that he does not know what to do. Gives me a bill and sends me down the road. I have now relocated twice with no Check Engine light coming on. Nest trip is going to be remote. Northern AZ and NM. Almost afraid to get on the road. Anyone have any similar experience, or any Check Engine light problems?