My 2003 gmc dura max won't start and I get a U1800 code.
And also a P090 code as well.
I've checked the wiring , checked voltage at the ficm. It has voltage and ground. Each glow plug has voltage, I've checked for air in fuel system. I've put an inline electric pump to lift fuel pressure.
These codes still pop up and I'm out of ideas of what to do.

Any help would be awesome!