Network, generic (extended)

U300A Ignition Switch

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U300A is a module's complaint about the ignition switch as a whole: the position or key information it receives does not add up, so it cannot be sure whether the driver wants off, accessory, run or start. The failure-type byte narrows it down. Ford's SSM 52606 covers 2023–2024 Super Duty trucks with a keyed ignition that display Vehicle May Not Start/Restart Service Now with U300A:01 stored by the body control module; a grommet around the lock cylinder had shifted and hindered the key, and reseating it is the repair. On Ram's 2025 heavy-duty pickups the LED headlamps, i.e. smart lamp modules left and right, log U300A-29, signal invalid, as the key moves from run to start. Ram says to leave it alone if it only comes back as stored, but to check the hardwired ignition line if it is active 10 s after start.

Likely causes

Key travel hindered at the lock cylinder, e.g. by a displaced grommet or trim
Module software that flags the run-to-start transition as invalid
Worn or dirty contacts inside the ignition switch
Open, chafed or corroded hardwired ignition line or connector

Symptoms

  • Start or restart warning in the cluster, possibly a no-start
  • Stored code in a lamp or body module with no noticeable effect
  • Key turns stiffly or does not reach every position cleanly

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–450
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ignition switchignition wiringbody control module

Related codes

U300DU3010U3011P2533U0140

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.