How an automatic door behaves when something is wrong
A working automatic door is invisible — you walk towards it, it opens, you walk through, it closes. A failing automatic door announces itself in specific ways, each of which points to a specific subsystem. Knowing the categorisation cuts diagnostic time from hours to minutes.
Five subsystems can fail: the activation sensor (sees approaching users), the safety system (sees users inside the swing/slide path), the drive train (motor, belt, gearbox), the controller (logic and timing), and the power supply (mains feed, isolators, internal PSU).
Symptom: door dead, no response
Power supply fault, controller lock-up, or activation sensor disconnected. In that order of likelihood.
Check: is anything on the controller showing power? LEDs, display, beeps. If completely dead — power feed gone, check the isolator and any inline fuse. If the controller is alive but unresponsive, look at the display for an error code, then try a power-cycle (off at the isolator, 30 seconds, on). Many controllers clear soft faults on power-cycle.
If the controller is alive and showing no error but still no response, the activation sensor is likely the issue. Look at the sensor face — most have an LED that lights when the sensor sees motion. Wave a hand in front; if no LED change, the sensor is dead or disconnected.
Symptom: door opens but will not close
Safety system is seeing something in the path, or the close command is being suppressed.
Look at the threshold first. Any debris — leaves, packaging, even a dirt patch — can trigger a safety beam. Clear. If the issue persists, look at the safety sensor lens (usually mounted above the door) — wipe with a dry cloth. Dust, condensation and spider webs are common culprits.
If safety is clear and the door still won’t close, check the controller for a stuck-open command (timer override, “hold open” mode, or “learn mode” engaged). Most controllers have a normal-operation indicator; if it shows hold-open, the door has been put into that state — often inadvertently via a key switch or via the controller buttons.
Symptom: door is slow, shuddering or noisy
Drive train issue. Belt, motor, gearbox, or top-track contamination.
Listen and look. A whining motor with reduced door movement = drive belt slipping. A grinding sound = gearbox wear. A juddering door on the slide = top track debris or worn carrier wheels. A shudder only at end-of-travel = limit switch or controller deceleration ramp issue.
Drive-train repairs are engineer-only. Belt re-tensioning is straightforward; motor or gearbox replacement is a 3–4 hour job; controller deceleration ramp adjustments require the engineer to be familiar with the specific brand’s programming menus.
Symptom: door reverses unexpectedly
Safety reversal — by design when something is in the swing path, by fault when nothing is. Required behaviour under BS EN 16005.
Test by triggering the safety system intentionally (hand in the path) — does the door reverse cleanly? If yes, the system is working correctly and the false reversals point to a contamination, misalignment or sensitivity issue with the safety sensor.
Common causes of false reversals: dirty sensor lens, sensor misalignment after vehicle impact to the frame, controller force setting too sensitive (door treats every micro-resistance as an obstruction), drive train wear creating resistance the controller misinterprets as obstruction.
Symptom: door operates intermittently — sometimes works, sometimes doesn’t
The hardest fault to diagnose. Almost always: marginal power supply, marginal sensor, or controller memory issue.
Marginal power supplies fail when ambient temperature changes, during high mains load events, or after extended runtime. Often resolved by power supply replacement — cheap part, big diagnostic benefit because intermittent faults are otherwise hard to pin down.
Marginal sensors fail in specific conditions: bright sunlight on the lens, condensation in cold mornings, particular vehicle types triggering false positives. Pattern-recognition by an engineer narrows it; sensor replacement is the typical fix.
After the repair: the force test
BS EN 16005 (the safety standard for power-operated pedestrian doors) requires force testing to verify the door cannot exceed safe contact force on a person — typically 150 N maximum closing force, 67 N for sustained force after stop.
After any repair affecting the drive, motor, controller or safety system, the force test must be re-performed and a fresh certificate issued. This is engineer-only work using a calibrated force gauge. Skipping the test means the door is operating outside its safety certification.
For maintenance contract customers, the force test is part of the scheduled visit. For ad-hoc repairs, ask the engineer to include the test in the quote — it should be standard, not an extra.