Part 3 — The ABS Fault Was the Missing Piece The dash eventually showed an ABS fault: Relay Diagonal 1 / Fault Code 802. That changed everything. A regular scanner did not show much because the problem was not really an engine code. It was in the ABS / brake control side. This is the important lesson: adaptive cruise and collision mitigation depend on the ABS and brake system. If the ABS system has a fault, the truck may disable adaptive cruise, collision mitigation, and sometimes regular cruise control too. So even though the dash was yelling about radar, the truck did not trust the brake system. I replaced the two ABS wheel speed sensors first. Right away, braking became smoother — no more jumpy or bumpy feeling. That confirmed the sensors were part of the problem. But the remaining issue pointed to the steer axle ABS relay/modulator assembly.