Service & test modes: Furnace tips

Short, practical technician tips on service & test modes for furnace. Check each one against the manual for your exact model before acting.

Read the last 7 faults (3-digit display furnaces)

Carrier · 59SC6A, Payne PG96VTA and 59MN7C furnaces (3-digit display control)

The FLt menu stores the 7 most recent faults in major.minor form; non means none stored.

1. Scroll the main menu to FLt with MENU/SELECT
2. Press NEXT/OPTION to step through the stored faults
3. Decode the codes with the furnace service label

Recall stored status codes (amber LED furnaces)

Carrier · 59TP6A and 58SB0A/58SB1A furnaces (amber status LED control)

Replays the stored status codes on the amber LED; codes survive power loss. Two-digit codes are short flashes (first digit) then long flashes (second digit).

1. 59TP6A: no thermostat signal, blower-off delays finished, 115 V on, blower door removed
2. 59TP6A: turn SW1-1 ON and hold the blower door switch closed; up to 7 codes flash, the 8th is always 11 (nothing stored)
3. 59TP6A: turn SW1-1 OFF, refit the door; steady amber LED = normal
4. 58SB0A/58SB1A: with no call and delays finished, note the LED state through the sight glass
5. 58SB0A/58SB1A: briefly pull the wire off the draft safeguard (DSS) switch until the LED goes out, then reconnect; the stored code flashes 4 times (11 = none)
6. 58SB0A/58SB1A: do not leave the LS or DSS wire off longer than 4 seconds or the blower starts and the stored code is replaced by 33

Run the component test from the menu (3-digit display furnaces)

Carrier · 59SC6A, Payne PG96VTA and 59MN7C furnaces (3-digit display control)

Starts the self-test from the Ct menu item; only starts from idle (iDl) with no W, Y or G demand.

1. Disconnect the thermostat wire at the R terminal so no demand is present
2. Hold the blower door switch in with one hand to power the board (do not tape it down)
3. Scroll the main menu to Ct and press MENU/SELECT; the sequence runs by itself (tSt, PUr, HSi, Fn, End)
4. Reconnect R and refit the blower door
5. Err on the display means the test could not start: clear thermostat inputs or faults and make sure status is iDl

Run the furnace component self-test (amber LED furnaces)

Carrier · 59TP6A and 58SB0A/58SB1A furnaces (amber status LED control)

Runs inducer, igniter and blower in sequence with no call for heat so each part can be checked. Only starts with no thermostat signal; the gas valve never opens.

1. 59TP6A: remove blower door and the R wire; turn SW1-6 ON and hold the door switch closed
2. 59TP6A sequence: inducer high-heat speed; after 10 s igniter on 15 s; blower mid-range 15 s; inducer low-heat speed 10 s, then stops
3. 59TP6A: turn SW1-6 OFF, refit R wire and door; status code 11 or 25 flashes afterwards; to repeat turn SW1-6 OFF then ON
4. 58SB0A/58SB1A: disconnect R lead, hold door switch closed, jumper COM-24V to the TEST/TWIN quick-connect until the LED goes out (about 2 s)
5. 58SB0A/58SB1A sequence: LED repeats previous code 4 times; inducer runs to the end; igniter 15 s; blower HEAT speed 10 s then COOL speed 10 s; inducer stops
6. 58SB0A/58SB1A: reconnect R lead, release the switch, refit the door

Run the R-32 leak function test (A2u)

Daikin · Daikin DR96TN/DD96TN two-stage furnaces

Simulates an R-32 leak so you can check the furnace response; it only works when no alarm or fault is active.

1. Press the left or right switch until A2u shows
2. Press the center switch, choose yes with the left or right switch
3. Press the center switch to confirm and start the test
4. Select no in the same menu to stop it early

Code release number menu

Goodman · All 34.5in chassis furnaces

Reference only. Shows the firmware release revision for each micro-controller.

1. Enter the code release menu to read the firmware revision of each micro-controller.

Gas pressure test mode (100% firing)

Goodman · All 34.5in chassis furnaces

The gas pressure test menu locks the burner at 100% firing rate so gas valve pressure can be checked. Inlet supply pressure must match the unit rating plate with all other household gas appliances running.

1. Enter the Gas Pressure Test menu to lock 100% firing rate.
2. Measure gas valve pressure while the burner is at full fire.
3. Confirm inlet supply pressure matches the rating plate with other gas appliances running.
4. Leave the test mode when finished.

High fire and low fire gas pressure test (longer term)

Goodman · Gas pressure test, high and low fire

Forces a continuous high fire or low fire run using thermostat W. Remove the communicating or RJ12 connection first as described.

1. Turn the furnace off.
2. On a communicating fossil fuel system, remove or disconnect anything on terminals 1, 2, R and C. With a non-communicating heat pump module, unplug the RJ12 jack.
3. Power the furnace.
4. High fire: apply a W call. The system runs continuously at 100% firing. Remove W to shut down.
5. Low fire: apply W, let ignition start, then remove W so the furnace enters idle (display shows IdL).
6. Apply W again. Low stage runs for more than 5 minutes if the target run time is set above 10 minutes.
7. Remove W to shut down when done.

High fire gas pressure test (temporary)

Goodman · Gas pressure test, high fire

Runs high fire for a short time from the on-board buttons. The CoolCloud app is preferred for functional tests.

1. Use the on-board buttons to enter the Gas Pressure Test menu (display shows gPt).
2. Select YES to enable the test.
3. High fire runs for 5 minutes or until any of the 3 buttons is pressed.

Testing gas heat only with a heat pump installed

Goodman · Furnace with communicating outdoor unit

With a heat pump installed, the system may stage between heat pump and furnace during a heat test.

1. Disconnect communications between the indoor and outdoor units.
2. Run the heating test to check gas heat only.

View and clear the last 6 furnace faults

Goodman · GMVC95/AMVC95 upflow and *CVC9 counterflow two-stage furnaces

The furnace keeps its last six faults, newest first. The same fault repeating is stored only 3 times in a row.

1. Open the furnace advanced menus from the ComfortNet thermostat.
2. Go to DIAGNOSTICS: FAULT #1 is the newest, FAULT #6 the oldest.
3. Set CLEAR to YES to wipe the history. Clear it after you finish a service call.

Lock the furnace in a heat stage for testing (9MAE)

ICP · (F/G)9MAE0601714A, 0602120A, 0801714A, 0802120A, 1002122A, 1202422A

Setup switches and an R jumper hold the furnace in minimum, intermediate or maximum heat. Return SW1-2 and SW4-2 to OFF unless a 2-stage thermostat is used, and leave SW1-1 and SW1-6 OFF.

1. Minimum: SW1-2 ON, SW4-2 OFF, jumper R to W/W1
2. Intermediate: SW1-2 OFF, SW4-2 ON
3. Maximum: jumper R to W/W1 and W2
4. Afterwards return SW1-2 and SW4-2 to OFF (unless 2-stage thermostat)

Recall the last 7 stored status codes (SW1-1)

ICP · ICP (F/G)9MAE modulating furnaces

Setup switch SW1-1 ON plus the blower door switch makes the amber LED flash up to seven stored codes in order, then a heartbeat flash. Codes survive power loss.

1. No thermostat call present and all blower-off delays finished
2. Leave 115 V on and remove the blower door
3. Turn SW1-1 ON and manually close the blower door switch
4. Watch the LED: up to 7 codes flash, then a heartbeat
5. Turn SW1-1 OFF, release the door switch and refit the door

Run the furnace component self-test (SW1-6)

ICP · ICP (F/G)9MAE modulating furnaces

Setup switch SW1-6 starts a self-test: inducer, gas valve communication check, igniter 15 s, blower mid airflow 15 s, then inducer stops. At the end the LED flashes a result: heartbeat, 2+5, 3+5, 4+1 or 4+2.

1. Remove the blower door and take the wire off the R terminal
2. Turn SW1-6 ON
3. Manually close the blower door switch and watch the test run
4. Release the switch, reconnect R and refit the door; to repeat turn SW1-6 OFF then ON

Read the last stored fault code (9MXE / 9MXT)

KeepRite · (F/G)9MXE and (F/G)9MXT furnaces

The control remembers the latest fault codes through power loss. Briefly pulling one main limit wire makes the status LED flash the last stored code, then a component test starts. Does not work during a thermostat call or blower off-delay.

1. Leave 115 V on; end any thermostat call and wait out blower delays
2. Watch the status LED through the blower door
3. Open the control door and, for a couple of seconds only, pull one main limit wire off and put it back
4. Read the flashed code against the service label
5. A component test sequence follows; refit the door

Run the furnace component self-test (9MXE / 9MXT)

KeepRite · (F/G)9MXE and (F/G)9MXT furnaces

Runs inducer, igniter and blower speeds briefly; gas valve and humidifier terminal are not energized. Will not start with a thermostat signal or running delays. Holding the jumper longer than 2 seconds makes the LED flash code 10 and the request is ignored.

1. Remove the blower door, disconnect the thermostat R lead at the control and hold the blower door switch closed
2. Jumper terminal C to the TEST/TWIN quick-connect for about 2 seconds, until the LED goes off, then remove the jumper
3. The LED repeats the previous status code 4 times, then the inducer starts
4. 9MXT: igniter on after 7 s for 15 s; blower LO-HT, HI-HT, COOL speed 10 s each; inducer low for 10 s and stops
5. 9MXE: igniter on after 7 s for 15 s; blower continuous-fan, heat and cool speed 10 s each; inducer stops
6. Reconnect R, restore the door, confirm the furnace shuts down and restarts from the thermostat

Test or reset the Low GWP refrigerant detection board (Test button)

Lennox · Lennox ML180UHEK furnaces with Low GWP refrigerant detection control board

A short press in Normal mode simulates a leak response (E164 for about a minute); in Leak Detected or Fault mode it resets the board after the problem is fixed; while initializing it skips the rest of the pre-purge.

1. To test after installation: restore power and wait for the initializing purge to finish.
2. Start a cooling or heating call and press the Test button; the blower should start, the compressor should stop (and the burners on a heat call).
3. Press Test again to end the simulated leak.
4. After a real leak or fault is resolved, press Test to bring the board back to Normal.

Extended display mode

Rheem · (-)96VA040-115 (R96V / U96V) and (-)(-)96MDV040-115 two-stage variable-speed communicating furnaces

Pressing the pushbutton while the control powers up switches the displays to extended mode, which changes what is shown during operation.

1. Cut power to the furnace.
2. Hold the pushbutton and restore power.
3. Cycle power again to return to the normal display (the extended-mode table is not readable in the text copy of the manual; see printed p.96).

Fault history display (FL)

Rheem · 96V and 96MDV communicating furnaces

Up to six faults are stored, and the newest replaces the oldest. Faults older than 168 powered hours are deleted. Each fault code can be stored up to three times.

1. Enter the FL menu with the pushbutton status mode
2. Faults are shown one after another from newest to oldest, each for about one second with a short separator
3. The sequence repeats until the status menu is exited

Force high-stage gas heat for testing

Rheem · 96V and 96MDV series 2-stage communicating furnaces

Used to test high-stage gas heat with a communicating thermostat.

1. Remove the communication connections from the thermostat (E1, E2).
2. Jumper W1 to R and R to W2 to set high-stage gas heat.
3. Set the FAN switch to AUTO on the thermostat.
4. Set the thermostat to call for heat with a temperature differential above 10°F.
5. Note the dip switch settings before testing and restore them afterward.
6. Test mode times out after one hour; check that the thermostat does not switch to high fire or test mode is not timing out.

Force low or high fire for testing (RGPE/RGLE)

Rheem · Rheem/Ruud RGPE / RGLE modulating furnaces

Temporarily locks the gas firing rate and blower to low or high for the heat call already running, up to two hours or until the call ends. Will not engage without a heat call.

1. Start a heat call at the thermostat
2. Open SETUP in the furnace USER MENU and choose FIXED FIRE RATE
3. Select low or high; the setting lasts for this heat call (max 2 hours)

Read or clear the six-fault history from the thermostat (RGPE/RGLE)

Rheem · Rheem/Ruud RGPE / RGLE modulating furnaces

The FAULT HISTORY user menu shows the six most recent faults, newest first, with the number of powered days ago. The last item, CLEAR FAULTS (yES / no), wipes the list.

1. Open the furnace USER MENU at the communicating thermostat and select FAULT HISTORY
2. Press the down arrow to step to older faults
3. To erase, go to CLEAR FAULTS and pick yES

Carbon monoxide spillage test (vent check)

Trane · Draft hood appliances on a shared vent

Run before returning a furnace to service on a shared vent. Failure to follow this can cause carbon monoxide poisoning or death.

1. Seal any unused openings in the venting system.
2. Check the venting system for proper size, horizontal pitch, blockage, restriction, leakage and corrosion, per NFPA 54 / CSA B149.1.
3. Close building doors and windows and the doors between the space and the other appliances.
4. Close fireplace dampers.
5. Turn on clothes dryers and any appliance not on the vent. Run exhaust fans such as range hoods and bathroom exhausts at maximum speed. Do not run a summer exhaust fan.
6. Light the appliance under test and set the thermostat so it runs continuously.
7. After 5 minutes of main burner operation, test for spillage at the draft hood relief opening with a match or candle.
8. If spillage shows, correct the venting to code before continuing.
9. Return doors, windows, fans, dampers and other appliances to their previous condition.

Read or clear the last 6 faults (L6F menu)

Trane · S8V2, S8B1, S8X1, S8X2

The L6F menu replays the six most recent faults and clears them from inside.

1. Press Menu until L6F appears
2. Press Option to enter and step through the faults
3. To clear: inside the menu hold Option for at least 5 seconds, release; three dashes appear three times

Run test mode (furnace)

Trane · S8V2, S8B1, S8X1, S8X2

Lets the technician watch the inducer, igniter and blower step through stages without firing the burner or running the outdoor unit. Option holds a step (HLd). The test takes a short while to begin.

1. Press Menu until run appears, then press Option; the display flashes three times and the test starts
2. Steps: inducer 1st then 2nd stage (30 s each), igniter 10 s, then blower at 1st/2nd-stage compressor speed and 1st/2nd-stage gas heat speed (10 s each); the sequence repeats two more times
3. Exit by pressing Menu briefly, cycling furnace power, or making a valid thermostat call

Read or clear stored fault codes with the LAST ERROR button

York · York IFC boards with LAST ERROR button

The board keeps up to five fault codes, newest first, through power loss. Solid-LED errors are not stored. Retrieval only works with no thermostat signal active.

1. End any heating, cooling or continuous-fan call
2. Press and release LAST ERROR (more than 1/5 s, under 5 s); the LED flashes the stored codes, most recent first, with a 2 second pause between
3. TM9V: after the codes it returns to slow green flash after a 5 second pause; press again to repeat
4. Two green flashes = nothing stored
5. To clear, hold the button 5 seconds or more; three green flashes confirm