LiftMaster · Garage Door Opener Moderate

4-3 — Position sensing problem (door speed)

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.

Keep using it?
Yes, with caution
DIY difficulty
Easy
Verified against
support.chamberlaingroup.com ↗

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.

Why it happens

Typical signs: 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

  • Loose chain or belt Chain/belt slop allows trolley to lag or surge.
  • Worn trolley Internal carriage worn, causing inconsistent travel.
  • Slipping drive sprocket or coupler Worn motor coupler on screw drive openers.
  • Damaged RPM sensor magnet Encoder ring chipped or contaminated.

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.

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

More LiftMaster garage door opener codes

1-1 1-1 — Safety Sensor Obstructed, Misaligned, or Disconnected 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 — No RPM Detected at Start of Movement 1-6 1-6 — Unexpected RPM After Stop at Open

All LiftMaster codes → All garage door opener codes →

Frequently asked questions

What does the 4-3 code mean on a LiftMaster garage door opener?

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.

What causes the 4-3 code on a LiftMaster garage door opener?

Loose chain or belt: Chain/belt slop allows trolley to lag or surge. Worn trolley: Internal carriage worn, causing inconsistent travel. Slipping drive sprocket or coupler: Worn motor coupler on screw drive openers.

Is it safe to keep using a LiftMaster garage door opener with a 4-3 error?

Yes, with caution. This is rated a moderate-severity fault.

How do I fix 4-3 on a LiftMaster garage door opener?

Inspect chain or belt tension — should deflect about 1/2 inch under hand pressure at center of rail. Adjust the trolley tension nut per the manual to remove excess slack. Check the motor coupler (screw drives) for splits or wear — replace if cracked.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: support.chamberlaingroup.com.