Quick answer
4 flashes on a Goodman furnace means an open main or auxiliary temperature limit switch: the furnace overheated, the burners are locked off, and the circulator blower runs continuously to cool it down. Replace the filter and open every register. If 4 flashes returns with a clean filter, a technician needs to measure temperature rise, blower speed and duct static pressure.
When a Goodman furnace shows 4 flashes, a main or auxiliary temperature limit switch has opened because the furnace got too hot. The board shuts the burners off and runs the circulator blower continuously to carry the heat away, which is why the vents blow air that never warms up. The limit is doing its job; the question is why the furnace overheated. Goodman has used more than one control board, so read the code legend inside your own furnace door and trust it over any website.
Is it safe to keep running a Goodman on 4 flashes?
The burners stay locked off while the limit is open, so the furnace is not a hazard sitting in this state. The harm is in the repeats. Every overheat stresses the heat exchanger, and repeated overheating shortens its life; replacing a heat exchanger runs $1,500–$3,000, money that on an older furnace is better put toward a new one. Treat 4 flashes as a problem to solve this week, not a code to reset until spring.
The safe fix to try first: the filter and the registers
Low airflow is the usual cause of an open limit, and the filter is the usual cause of low airflow. A furnace can only get rid of its heat through the air the blower pushes across the heat exchanger.
- Pull the filter. If you cannot see light through it, replace it with the same size; the dimensions are printed on the frame.
- Open every supply register and uncover every return grille, including the ones behind furniture and in a finished basement.
- If you recently switched to a denser pleated filter, go back to the type the furnace ran on before. A filter that is too restrictive starves the blower the same way a dirty one does.
- Let the furnace cool and run a full cycle. If the heat comes back and stays, write the date on the new filter and set a reminder using our guide to how often to change a furnace filter.
What causes a Goodman limit switch to open
- Blocked filters. First on Goodman's own list, and the only cause a homeowner can fix.
- Restrictive ductwork. Closed dampers, crushed flexible runs, too little return air, or a basement renovation that boxed in a return grille.
- The wrong circulator blower speed. The heating speed is chosen at installation; a setting too low for the furnace's output lets the temperature climb on long cycles.
- A failed circulator blower. The board calls for the blower and nothing moves the air, so the heat has nowhere to go.
- A faulty limit or loose wiring. A limit that opens at normal temperatures, or a connection in the limit circuit that has worked loose, looks the same to the board as a real overheat.
House age gives a hint. In 1970s to 1990s subdivision homes across Burlington, Oakville and Mississauga, the original return ducts were sized for the furnace of the day, and a later furnace swap or finished basement can leave the new one short of air.
What a technician measures on a 4 flashes call
If 4 flashes returns with a clean filter, the answer comes from three measurements: temperature rise, blower speed and duct static pressure. Temperature rise is the difference between the air going into the furnace and the air coming out, and the allowed range is printed on the furnace's rating plate. A rise above that range with a clean filter means the blower setting or the ducts are holding the furnace back, and static pressure readings show which.
- Diagnostic visit: $90–$150, deducted from the repair if you approve it.
- Blower motor replacement: $500–$1,500, depending on the motor type.
- Limit switch replacement or a blower speed change: quoted in writing on the visit.
- Duct corrections such as an added return: priced after the static readings are taken.
Ask for the measured temperature rise and static pressure to be written on the invoice. Those two numbers tell the next technician, or the next owner of the house, whether the fix held.
Goodman is on the list of brands ZK Mechanical services, and our furnace repair work covers Burlington, Oakville, Milton, Hamilton and the west side of the GTA. The Goodman furnace error code guide covers the remaining flash counts. Call (647) 801-1252 or book online when a new filter has not cleared the code.