How to get get ddec iii fault codes

Is there anyway to pull fault codes on a 98 ddec iii? I have a over ride/check button right beside the start button but pushing this after turning ignition on doesn't do anything. Even tried the optional switch that doesn't seem to do anything either. The override/ check button wouldn't b disable somehow in the ecm, would they?
Is there any other way to active diagnostics without prolink or dddl? Maybe jump it through the plug? Or some kind of key dance like old cars back in the day?
Thanks for any and all advise

 

I really appreciate all the info you guys are giving. I'm heading back up to truck to try all recommendations.
heavyhaulerss you must have a different override button than me. Mine looks just like my starter button. But I'm fixing to examine. And bayberries try jumping just those 2 wires hooked to back. Any more info would be appreciated.
Be safe out there

 

You guys are great! Thanks for your time, knowledge, and wisdom. I pulled light panel above tach and speedo; the board has 2 capacitors that are burnt/ black board. I'm looking for replacement now.
If anyone knows which wire/pin on the back goes to cel light it be helpful in proving the bad board. And not anything else. Thanks.
Freightliner wanted $650 for new. I'll find a salvage yard

and wore out; she looks like something worth keeping up to me! But I love the old trucks