The three sensor jobs on an automatic door
A modern automatic sliding or swing door has three distinct sensing jobs, performed by separate sensors that are commonly conflated in conversation. First, activation: detect an approaching person and tell the operator to open the door. Second, presence: detect someone standing in the threshold and tell the operator to hold the door open. Third, safety: detect a moving object during the closing arc and tell the operator to re-open before the door makes contact.
A door that fails the first job will not open for users. A door that fails the second job closes onto users standing in the threshold. A door that fails the third job re-opens repeatedly because it is seeing imaginary obstructions. All three faults look different to the user and have different sensor sources.
See commercial automatic door repair for the parent silo and automatic door repairs — common faults for the user-facing diagnostic guide.
Activation sensor faults — door won't open
The user walks toward the door, the door does not open. Two technologies are common. Passive infra-red (PIR) sensors detect body heat against the background; they fail when contaminated by dust, when aimed at a moving heat source (e.g. a sunny floor or a radiator), or when the lens has yellowed with age. Microwave sensors detect movement against the background using doppler shift; they fail when interfering RF sources move close (e.g. a new till system, a microwave oven in a nearby room), when aimed at vibrating signage or banners, or when the sensitivity has been knocked down by a previous engineer.
The diagnostic routine: stand inside the threshold, wave at the sensor (rules out activation circuit failure), check the sensor LED for activity, check the wiring to the operator. False-negative activation is far more often a sensitivity-adjustment or alignment fix than a dead sensor. We replace the sensor only after the adjustment route has been exhausted.
Presence sensor faults — door closes on users
The user is standing in the threshold, the door starts to close. Presence sensors are usually safety-laser-curtain (LZR or similar) units mounted at the top of the door frame, pointing down into the threshold zone. They project a fan of infrared laser beams and detect any obstruction inside the fan.
Failure modes: contamination of the sensor window (dust, fingerprints, water film), de-alignment after physical impact, or end-of-life on the sensor itself (typical service life 7–10 years on a heavy retail door). The diagnostic routine: walk slowly through the threshold and watch the sensor LED, then check the calibrated detection zone in the sensor's diagnostic mode.
This is the single most safety-critical sensor on an automatic door — a presence-sensor failure can injure users. We treat presence-sensor callouts as same-day urgent and recommend immediate isolation of the door to manual operation until the repair is completed.
Safety sensor faults — door re-opens for no reason
The door closes, then re-opens. Closes, re-opens. Closes, re-opens. The safety sensors in the door leaf (small windows at the leading edge of each panel) are detecting a phantom obstruction that does not exist. Causes: leaf-mounted sensors aimed too far into the closing arc and picking up the door frame on the opposite side. Sunlight hitting the sensor at certain angles. Dust or rain on the sensor window. A recently-changed sensor that was not re-zoned during fitting.
The diagnostic routine: walk away from the door and watch the closing cycle, identify what is in the sensor's view at the moment the re-opening triggers. False re-opens have a clear environmental cause in roughly 80 per cent of cases — sensor replacement is the right call only for the remaining 20 per cent.
The diagnosis-first approach saves money
Automatic door sensors are not cheap parts. A genuine Bea, Optex or Hotron sensor runs to several hundred pounds. Replacing a sensor that turns out to be working but mis-adjusted is wasted spend.
Our routine on any automatic-door sensor callout is to diagnose first, replace second. The engineer's first 30 minutes on site are spent on adjustment, alignment and environment checks. If the sensor itself is the fault, replacement follows; if the adjustment route fixes the symptom, we report what we found and reset the door without changing parts.
Sensor replacement work
A single-sensor replacement on a standard sliding door is a 1–2 hour job. The relevant operator is isolated, the failed sensor is unbolted from the housing, the new sensor is fitted and wired, the detection zone is calibrated, and the safety cycle is tested across the full closing arc. We test with a person walking through, then with a test card, then with a long thin object (broom handle) to confirm the sensor sees small obstructions as well as large ones.
A full sensor refresh (activation + presence + safety on a sliding door pair) is a half-day to full-day job. We schedule out-of-hours for retail and hospitality customers to keep the door fully working during trading hours. See automatic door repair and drive arm repair for parent services.
Preventing sensor failure
A quarterly sensor check is enough to catch most failures in the cheap-fix window. The check covers detection-zone calibration, sensor-window cleaning, alignment verification and a written test report. Automatic door service contracts include this work, and most multi-site retail and healthcare customers are on contract because the safety obligation makes ad-hoc maintenance impractical.
See also automatic sliding door repairs for the related repair work on the operator and drive mechanism, and healthcare automatic doors for sector-specific compliance.