The limit is a thermometer with authority. It opens on temperature, and temperature climbs when air stops moving heat away from the exchanger. Trane's guide entry is four words — "open temperature limit circuit" — because the causes are the standard airflow litany and the manual trusts you know it: filter, duct restrictions, a failing blower.
Do the airflow list in full, not in spirit: filter out and inspected against the light, every supply register open, returns unblocked, and the blower actually running during a call (a furnace that heats with a dead blower posts this code fast, and that version is a motor call, not a filter).
Homeowner checks — the safe list:
- Air filter first. On limit codes the filter is the cause often enough that a technician will ask about it before anything else.
- Walk the house and open every supply register. Closed or blocked registers strangle airflow.
- Check the returns: rugs, furniture, and storage boxes parked against return grilles count.
- If the code comes back on a clean filter with everything open, the blower or the limit switch itself needs a technician.
4 flashes: Open temperature limit circuit
Homeowner checks
The furnace overheated and the limit opened the circuit. Airflow first, always.
From the table: Red-LED flash codes — integrated furnace control — source: Trane UC Series Installer's Guide 18-CD19D5-10, diagnostic codes (read the document, fetched 2026-10-30). Covers: Trane/American Standard furnaces with the IFC covered by Installers Guide 18-CD19D5-10 (UC/AUH families and relatives). Green LED reads system state; the red LED flashes the faults.
When 4 is not the filter
A 4 that returns within a single cycle on a clean filter, with the blower running, moves the suspicion to the limit switch itself or a genuinely over-fired furnace — both meter-and-gauge diagnoses. And if the open device in the chain is a roll-out rather than the main limit, the calculus changes entirely: that is flame where it should not be, and it gets a technician regardless of how clean the filter is.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.