Quick answer
Code 3 on a KeepRite furnace means the pressure switch did not close or reopened; on modulating models it refers to the low pressure switch. The furnace will not fire without proving draft, and the switch must never be bypassed. Clear snow and ice from the intake and exhaust terminations and check that the condensate drain is not frozen. If code 3 continues, call a technician.
Code 3 on a KeepRite furnace means the pressure switch did not close, or closed and then reopened. On modulating models the code refers to the low pressure switch. Either way the board has not been able to prove that the inducer is moving combustion air through the furnace and out the vent, so it keeps the gas valve shut. The status code legend on the inside of the furnace door is the reference for your model and overrides any general list.
Why a KeepRite shows code 3 and not Carrier's code 31
KeepRite furnaces are built by International Comfort Products (ICP), the Carrier-owned company that also makes Tempstar and Heil. ICP control boards report this fault as a single digit, so a KeepRite owner searching Carrier's two-digit list will not find a match. The equivalent pressure switch fault on a Carrier-branded furnace is code 31. If a guide tells you to look up a two-digit number on a KeepRite, it was written for a different board.
Is code 3 safe, and what can you check yourself?
A furnace on code 3 will not fire without proving draft, which is the safe outcome. Do not bypass the switch to get heat: it is the one device confirming that exhaust is leaving the house.
- Outside, find the intake and exhaust terminations and clear snow, ice and debris from both. After freezing rain, look for a cap of ice over the exhaust opening.
- Inside, check the condensate drain line for a frozen section, a kink or a full pump tank.
- Note the weather. Code 3 that appears only in strong wind and clears when the wind drops is a clue the technician needs; see why a furnace shuts off when it is windy.
- Leave the pressure tubing, the trap and the vent joints alone.
Did not close versus reopened: the difference matters
The wording of code 3 covers two different failures. A switch that never closes at startup points to something constant: a blocked termination, a plugged drain, tubing that has come off, or a switch that has failed. A switch that closes, lets the furnace light and then reopens points to something that changes while the furnace runs: water collecting in a sagging section of vent pipe, a gust against the termination, or condensate backing up as the furnace makes it.
Tell the technician which one you saw. A furnace that heats for a while and quits has a different problem from one that never lights. Note too whether the inducer was still running when you found the furnace stopped.
Causes of KeepRite code 3, from the manufacturer's list
- A restricted vent: snow, ice, a nest or debris at the termination.
- A plugged condensate drain.
- Water in the vent piping, where a horizontal run has sagged and now holds a pool.
- Disconnected or obstructed pressure tubing.
- Excessive wind. Houses on open lots, along the Lake Ontario shore or near the brow of the Niagara Escarpment take more gusts at the vent than houses sheltered mid-block.
- Improper vent sizing. Pipe too small or a run too long for the furnace is an installation fault that shows itself on the coldest, windiest days.
- A defective or miswired pressure switch.
What a technician does for KeepRite code 3, and the cost
When the vents are clear and code 3 continues, the furnace needs a manometer and an inspection of the trap, the tubing and the inducer. The gauge shows how much draft the inducer produces against what the switch needs, the trap is opened and flushed, and the vent is checked for slope along its whole length. A weak draft reading with a clear vent and an empty trap turns attention to the inducer. Re-supporting or re-pitching vent pipe falls under the CSA B149.1 gas code and belongs to a TSSA-licensed technician.
- Diagnostic visit: $90–$150, taken off the repair bill if you go ahead.
- Condensate drain cleaning or trap replacement: $150–$350.
- Pressure switch: $250–$500.
- Draft inducer motor: $400–$900.
- Vent re-pitching or resizing: written quote after the inspection; see furnace vent pipe repair.
ZK Mechanical services KeepRite furnaces from Burlington through Oakville, Milton, Georgetown, Hamilton and Mississauga. For a code 3 that outlasts a cleared vent, book furnace repair at (647) 801-1252 or online. The full status code table is in the KeepRite furnace error code guide.