Engine light on,both of them .

How far from the mechanic are you. Can he hook up a laptop and clear the code.
If not. Disconnect the negative battery terminals for a couple minutes, at least 30 seconds. See if this will clear the code. There may be a way to key on without starting truck that may help also, not sure.
I'm just guessing. I don't have your type of truck.
Call a Freightliner dealer
Good luck.

 

Freightliners usually have a 'nighttime' switch in the cab, so you can disconnect power without having to get into the battery compartment.

 

I have a 99 FLD120 . Where might this ,nighttime, switch be.

 

Yours doesn't have it. They only put those in for a few years, they've stopped now, or at least they don't put them in every model. I don't think they shutdown the primary ECM anyway.

 

Honestly probably reflects badly on my part but i mostly ignore check engine lights on crapcadias. Ive had 3 of those pieces of garbage- 2015 70k miles, and 2 brand new 2016 9k and 2016 1200 miles. All three of them constantly have the check engine light coming on. Im not spending all of my working hours in the shop. I dont get paid for that.

 

That switch, if you have one, will be on the floorboard on the left side of the drivers seat. You can turn if off for 5 minutes, then back on. If your lucky, that may clear the codes.

 

Last time that happened on my 2010 it threw a egr fault code. I jiggled the rod connected to the egr and the lites went off.

I have cleared em by turning off that master switch mentioned above also. Good luck.

 

Yeah, it's a feature on more current models. It's basically a master disconnect, so, if you wanted to install one, you could do it quite easily.