Quick answer
Code 4 on a KeepRite furnace means a limit circuit fault: the limit switch or a flame rollout switch is open. The blower runs for 4 minutes or until the switch closes. Replace the filter and open all registers. If the switch stays open longer than 3 minutes, the furnace goes to lockout code 7, which resets automatically after 3 hours. A rollout switch needs a technician.
If you are seeing code 4 on a KeepRite furnace, the limit circuit is open: either the high-temperature limit switch or a flame rollout switch has tripped. The board shuts the burners off and runs the blower to cool the furnace. Most often the cause is an airflow problem that a new filter fixes; sometimes it is a rollout trip that must not be reset by the homeowner. Status codes differ by board, so read the legend on the inside of the furnace door and treat it as the authority.
What happens during code 4, and when it becomes code 7
The sequence is timed. With the limit circuit open, the blower runs for 4 minutes or until the switch closes, whichever comes first. If the circuit stays open longer than 3 minutes, the board escalates to lockout code 7, and that lockout resets automatically after 3 hours. A code 4 that clears in a minute or two and a code 7 on a cold furnace are the same problem at different stages.
Is it safe to keep running a KeepRite on code 4?
The burners stay off while the circuit is open, so the furnace is not firing in an unsafe state. A limit that trips once after a filter was left too long is a warning; a limit that trips every day is overheating the heat exchanger a little each time. A flame rollout trip is in a different category. It means flame came out of the burner area, the switch stays open until it is manually reset, and resetting it is not a homeowner job.
You can tell the two apart by what the furnace does next. If it relights on its own once the blower has cooled it, the automatic limit opened and closed again. If it never relights, even after the 3-hour lockout has timed out, suspect the rollout switch or a failed part, leave the furnace off and call.
The safe homeowner step: filter and registers
- Replace the furnace filter with a clean one of the same size.
- Open all supply registers and move furniture, rugs and boxes off the return grilles.
- Check that the blower door is seated and nothing is stacked against the furnace cabinet.
- Let the furnace run. If code 4 stays away through a cold night, the filter was the cause.
If the air at the registers has felt weak for a while, that history is worth passing on; why airflow from the vents is weak lists what else restricts a duct system.
Causes of KeepRite code 4, most likely first
- A dirty filter or a restricted duct system. Too little air crosses the heat exchanger, and the limit opens to protect it.
- A loose blower wheel. The motor spins but the wheel slips on the shaft, so airflow drops while the furnace sounds roughly normal; a rattle or scrape from the blower compartment is the hint.
- A defective switch or connection. A limit that opens early, or a loose wire in the limit circuit, mimics an overheat.
- Wrong gas input adjustment. A furnace set to burn more gas than its rating makes more heat than the blower can carry away, and correcting it is licensed gas work.
When to call about code 4, and what the visit costs
Call if code 4 returns with a clean filter, or if it changes to code 7. The technician measures the temperature rise across the furnace against the range on the rating plate, checks the blower wheel and speed, tests each switch in the limit circuit, and verifies the gas input. If the rollout switch is the one that opened, the reason flame left the burner area is found before anything is reset.
- Diagnostic visit: $90–$150, and it comes off the repair total when you approve the work.
- Blower motor, if the motor and not the wheel has failed: $500–$1,500.
- Gas valve, if the input cannot be set correctly: $500–$1,000.
- Limit switch, blower wheel or wiring repair: written quote on the visit; see furnace limit switch replacement.
KeepRite is one of the brands ZK Mechanical services. Book furnace repair in Burlington, Oakville, Milton, Hamilton or Brampton by calling (647) 801-1252 or sending a request. Related reading: the KeepRite furnace error code guide and, for the draft-side fault, KeepRite code 3.