New to this site, new to working on diesels. Now that the weather here in the PNW is turning agreeable, I'm tackling this nagging issue with my truck. I've done a lot of research on it, just looking for some prioritization of what/how to check first.

Issue is when I am on the throttle for more than 7 seconds, the CEL comes on with P1211. ICP looks fine, no oil. Truck has 144K. I was told by PO that the injectors were done 20K ago, but who knows. HPOP has to be original. When CEL comes on, noticeable drop in power, if I clear it, noticeable improvement. Very consistent, and it gets better the warmer the truck gets.

I used a ScanGuage II to get some ICP/IPR numbers. If I hold constant acceleration so that IPR hits 54.9 % and hold it there for 7 seconds, 3 things happen simultaneously. CEL light comes on, IPR drops to about 32% and ICP goes from about 1700 to 23-2400. Max IPR reading I ever get is 54.9 and max ICP is about 2500. Note, it is not just hard acceleration, I have a long hill and I can not make it up that hill without the CEL no matter how easy I am on the throttle.

I suspect a leak in the HP system, maybe injectors? Or weak HPOP, or IPR. What should I check first?

Thanks!