Retrieve diagnostic codes 1998 Fleetwood Discovery 5.9L Cummins on Freightliner

[WindsorE]

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 can occur after highway travel when 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. When idling, there is no rev, sputter, etc. Then, when the speed comes back up, it seems that the power loss is resolved and the check engine light goes off.
• There are 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:

1. Where can I find the port for diagnostics?
2. Is there a "dash flash" method for retrieving the codes?
3. What is the function of the two rocker switches (image)?
4. Advice on where to begin troubleshooting this issue.

Thanks in advance!

[BFlinn181]

It depends on when in 1998 your engine was made. That was the year the 5.9 12 v mechanical engine was phased out. In place of the 12v was a 24 valve ISB electronically controlled injection engine. The 24 v probably has a port somewhere to read codes. The 12 v doesn't have the capability to store codes. If it's check engine light it on, it's usually a fluid level, temperature sensor, etc.

The 'idle inc/dec' switch could be a way to up the idle speed. On my engine, the cruise control switched do the same thing. I can start and using the CC raise the idle to 1000-1200 RPM to air up faster. Try holding the top of the switch down and see if idle increases, it should slow down holding the bottom part down. It will normally cancel out once you hit the brake treadle.