4-3 Moderate Easy DIY

LiftMaster Garage Door Opener 4-3 — Position sensing problem (door speed)

LiftMaster Garage Door Opener · Error code 4-3

What 4-3 Means

Door is moving faster or slower than expected during travel — the opener cannot reconcile RPM data with expected travel and stops to protect the door.

Symptoms

  • Door starts then stops unexpectedly
  • Up arrow blinks 4, down arrow blinks 3
  • Door free-falls a few inches on opening
  • Loud chain or belt slap noise

Common Causes

1
Loose chain or belt

Chain/belt slop allows trolley to lag or surge.

2
Worn trolley

Internal carriage worn, causing inconsistent travel.

3
Slipping drive sprocket or coupler

Worn motor coupler on screw drive openers.

4
Damaged RPM sensor magnet

Encoder ring chipped or contaminated.

How to Fix It

  1. Inspect chain or belt tension — should deflect about 1/2 inch under hand pressure at center of rail.
  2. Adjust the trolley tension nut per the manual to remove excess slack.
  3. Check the motor coupler (screw drives) for splits or wear — replace if cracked.
  4. Inspect trolley for cracked housing or worn carriage.
  5. Run a learn/limit cycle to reset travel.
  6. If 4-3 persists with everything mechanical good, replace the travel/RPM module.

Parts You May Need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

Common replacement Motor coupler (screw drive) View on Amazon→ Common replacement Trolley assembly View on Amazon→

More LiftMaster Garage Door Opener Codes

1-1 1-1 — Safety sensor wire short or reversed 1-2 1-2 — Safety sensor wire shorted or reversed 1-3 1-3 — Door control or wire issue 1-4 1-4 — Safety sensors misaligned 1-5 1-5 — Motor overheated or stuck 1-6 1-6 — Logic board or motor relay failure

All LiftMaster codes → All garage door opener codes →

Verified against manufacturer documentation: liftmaster.com

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.