Carrier Understanding Furnace Fault Codes: Error Code 43 and Error Code 33

43.	If possible, replace inducer wheel. Otherwise, replace inducer motor assembly.	Ð	Ð	8
44.	Is there any frost build-up on combustion-air inlet?	45	46	Ð
45.	The problem is caused by reversed vent and combustion-air pipes or some other moist air entering	Ð	Ð	14
	combustion-air pipe.
→46.	You have inadequate combustion-air supply. This may be caused by:
	- Excessive restriction in combustion-air or vent pipe.	Ð	Ð	14
	- Internal restriction in coupling box or secondary heat exchangers
→47.	Replace pressure switch assembly.	Ð	Ð	70
→48.	Is unit a trophy unit?	50	49	Ð
→49.	Does fault occur before blower motor turns ON?	47	51	Ð
	Replace pressure switch assembly with pressure switch assembly used in NON-trophy models or
→50.	replace furnace control board. If you replace furnace control board make sure replacement control	Ð	Ð	70
	has a date code of 9837 or later. Only replace the pressure switch assembly or the furnace control
	board NOT both.
→51.	Replace furnace control board.	Ð	Ð	70
52.	Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals.	Ð	Ð	53
53.	Turn power on and wait several sec before proceeding to next step.	Ð	Ð	54
54.	Disconnect collector box tube to pressure switch assembly.	Ð	Ð	55
55.	Jumper R and W/W1 thermostat terminals on furnace control board.	Ð	Ð	56
56.	Does Fault Code 42 occur within 11 sec after a call for heat?	57	58	Ð
57.	Reconnect collector box tube to pressure switch assembly.	Ð	Ð	10
58.	Reconnect collector box tube to pressure switch assembly.	Ð	Ð	36
59.	Go to page number indicated in Index for CLEANUP AND STARTUP INSTRUCTIONS. If problem	Ð	Ð	INDEX
	persists on an intermittent basis, contact your distributor.
60.	Turn power off and disconnect jumper wire across R and W/W1 thermostat terminals.	Ð	Ð	61
61.	Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-	Ð	Ð	62
	nace control board.
	Does fault occur a few sec after high-pressure switch HPS makes?
62.	NOTE: Check for 24v between the N.O. (Normally Open) contact on high-pressure switch	63	67	Ð
	HPS and Com on furnace control board to know when high-pressure switch HPS makes.
63.	Turn power off and disconnect jumper wire across R, W/W1, and W2 thermostat terminals.	Ð	Ð	64
64.	Connect 1 side of a slope manometer with a tee to collector box pressure tap. Connect other side	Ð	Ð	65
	with a tee to gas valve air pressure fitting. Refer to pressure check diagram in Appendix C.
65.	Turn power on and wait several sec, then jumper R, W/W1, and W2 thermostat terminals on fur-	Ð	Ð	66
	nace control board.
	Do you have more than 1.9 in. wc pressure drop across heat exchangers when high-pressure
66.	switch HPS makes?	47	44	Ð
	NOTE: Check for 24v between the N.O. (Normally Open) contact on high-pressure switch
	HPS and Com on furnace control board to know when high-pressure switch HPS makes.
→67.	Does fault occur a few sec after main burners ignite?	63	68	Ð
→68.	Observe operation of furnace in high heat for next 8 minutes or until fault occurs.	Ð	Ð	69
→69.	Does the fault occur?	22	59	Ð
→70.	If problem still persists check for internal restriction in coupling box or secondary heat exchangers.	Ð	Ð	14

Fault Code 43

PRESSURE SWITCH CALIBRATION FAULTÐIndicates low-pressure switch LPS and high-pressure switch HPS "make" points during high-heat purge are not within calibration range. This is determined by reading inducer motor RPM when low-pressure switch LPS and high-pressure switch HPS make. Then the following comparison is made:

0.55¬ RPMRPM (high)(low) ¬ 0.90

If this relationship is not within range specified, furnace control board starts flashing fault colde 43, shuts unit down (inducer runs for 15 sec), waits 2 minutes, stops flashing fault code, and restarts heating cycle.

→It can also indicate that the high-pressure switch HPS made before the low-pressure switch LPS during either low-heat or high-heat purge.

STEP	ACTION	YES	NO	GO TO
1.	Turn power off, remove blower access panel, and disconnect all thermostat leads from furnace con-	Ð	Ð	2
	trol board.
2.	Turn power on and depress door switch. Use a piece of tape to hold switch closed.	Ð	Ð	3
3.	Jumper R, W/W1, and W2 thermostat terminals on furnace control board.	Ð	Ð	4

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