Failure mode - Ford 7.3L Power Stroke Diesel (T444E)

Glow plug / glow plug relay failure

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

extended cranking or no-start below ~40F, runs fine once warm
relay clicks but no voltage at the plugs (or never clicks)
white smoke while cranking cold; plugs test open

Compare with the other causes of the same sign on this engine: hard start, long crank or no-start (7 causes) · white smoke from the exhaust

Root cause

Heui engines need glow assist below ~40F. The single heavy relay (GPR) and the plugs are wear items; a dead relay strands the truck on the first cold morning. Harness/UVCH faults read identically - test before parts.

Diagnosis

measure voltage at the glow-plug bus while the relay is commanded (relay test)
ohm each plug through the UVCH: ~0.1-2 ohms good, open = dead plug or harness
check the relay's control circuit (key-on cold) before replacing parts

Fixes

replace the glow plug relay (upgrade relays common; carry the old one as a spare) ()

replace glow plugs (motorcraft/well-made only; cheap plugs swell and break off) ()

7.3L Power Stroke Glow Plug (OEM)

verify the UVCH/glow-plug circuit path while in there ()

uvch-harness - more than one option: 7.3L Power Stroke Internal Injector Harness 1994-1997 (aftermarket), 7.3L Power Stroke Under-Valve-Cover Harness 1999-2003 (OEM)

Prevalence

very-common

Parts for this repair run about $55-$160: see them next to the other repairs on this engine in repair costs.