2013 Cascadia won't idle past 1000 rpm

I have a 2013 Cascadia DD15 that I just got back from a shop.

They replaced the Air Compressor, which I can't imagine would be related to this. Anyways, the first compressor they installed did not work. They said it was a manufacturer defect, so they had to order a second Air Compressor which they installed today.

The truck builds air fine. But they gave it back to me in a condition such that it will not rev past 1000 rpm. And then, it idles around 725 rpm rather than the usual 600.

I tried killing the power with the battery switch, restarting, but it does the same.

The "check" light is on as is the solid yellow light. However, the solid light had been on forever already.

A few Google searches leads me to conversations about DEF issues. However, this truck IS deleted. I bought the truck in 2019 and it had been deleted by previous owner and this is the condition I have driven it in for the three years since then with no issue.

Out of curiosity I bumped the region switch and it started to idle up, but I cut the truck off because I didn't know if it's bad for it to send a deleted truck into regen.

Could this in anyway be related to the job that was performed? He said it was not doing this during the time they were running the truck after the first air compressor was installed. Are there any sensors that could have been knocked loose? They did have the drivers side fairing removed in-front of the batteries. There was at least one mechanic who seemed knowledgeable, but the main guy working on mine was a 22 year old night tech.

In the morning I will try to get somewhere I can read the codes. I will ask this shop to do it, but it has been like pulling teeth any time I have a question for them. This shop has been absolute losers. Today they replaced a flat tire on a fourwheeler car belonging to a elderly man in a wheelchair. They put his old tire in the trunk. For 10 minutes this man sat in his wheelchair and tried to wrestle the tire out of the trunk so he could put it in the back seat and then put his wheelchair in the trunk. I had to go out and help him because several shop employees simply walked past. Every single person here including the service dept manager has a dont-ask-me, no-can-do BULLXXXX Attitude.

I'm almost suspicious they did something to the truck on purpose because maybe they thought I was being too pushy and asking for updates. No way?

 

Thanks. I located the QCV and verified it was plugged in.

UPDATE:

One of the techs at this shop was nice enough to run the CODES and we got ONE ACTIVE CODE:

SPN 164 FMI 5

I called a freightliner and the service tech there told me this points to the FUEL RAIL SENSOR either being bad or a wiring issue.

I unplugged the fuel rail sensor for a few minutes, inspected it, and plugged back in. I started the truck and it idled at the correct rpm with a slight lope that gradually smoothed out. CEL did not reappear. I was able to idle the truck for about 10 minutes with no codes reappearing. The motor was also able to rev above 1000rpm.

It seems to be "better," however it does not rev smoothly. Revs with a "bump" around 900rpm. It still feels derated in some manner.

Any other advice is appreciated. For now, I will wait a little while and go back later on and idle it again and possibly drive it.