58SU0

Memory empty or power was cycled (two-digit boards).

1. 1 Confirm this is not a fault

   On the Carrier 58SU0 this readout (11) is run status, not a lockout. Write what the display actually shows and whether W/Y/G is present at the IFC.

   Expect: Idle / Ht / CF / stage+CFM words change when you apply and remove the call.

   If fail: If the word never changes on a real call, the call is not reaching this IFC.

   If pass: The board is alive. Diagnose the comfort complaint, not this code.

2. 2 Prove the call at the board

   Measure R to W / Y / G at the Carrier 58SU0 IFC, not at the thermostat. Communicating plates need the data pair and a live indoor address — 24 V on W is not the test.

   Expect: Not a fault. Recreate the W call.

   If fail: Fix the stat, harness, or data pair before you change parts.

3. 3 Watch one full sequence

   Leave power on. Watch Two-digit or vintage LED-status IFC (Comfort 80) through one cycle and write the first real fault that replaces this status word.

   Expect: W: Inducer → metal-vent draft switch must close (31 if not) → HSI → single-stage valve → flame (34 if no prove) → heat-speed blower.

Powered up on a W call.

1. 1 Confirm this is not a fault

   On the Carrier 58SU0 this readout (12) is run status, not a lockout. Write what the display actually shows and whether W/Y/G is present at the IFC.

   Expect: Idle / Ht / CF / stage+CFM words change when you apply and remove the call.

   If fail: If the word never changes on a real call, the call is not reaching this IFC.

   If pass: The board is alive. Diagnose the comfort complaint, not this code.

2. 2 Prove the call at the board

   Measure R to W / Y / G at the Carrier 58SU0 IFC, not at the thermostat. Communicating plates need the data pair and a live indoor address — 24 V on W is not the test.

   Expect: Normal after a brownout on two-digit IFCs.

   If fail: Fix the stat, harness, or data pair before you change parts.

3. 3 Watch one full sequence

   Leave power on. Watch Two-digit or vintage LED-status IFC (Comfort 80) through one cycle and write the first real fault that replaces this status word.

   Expect: W: Inducer → metal-vent draft switch must close (31 if not) → HSI → single-stage valve → flame (34 if no prove) → heat-speed blower.

Hardened 33. Limit/rollout stayed open.

1. 1 Treat this as flame outside the exchanger

   Do not reset the Carrier 58SU0 rollout and walk away. Write the code, kill the call, and look for flame at the burners, crossover, and vestibule before you do anything else.

   Expect: No flame outside the exchanger. Burners seated. Crossover lit.

   If pass: Still inspect secondary / flue / condensate before you reset.

2. 2 Secondary, condensate, and flue

   Blocked secondary, drowned collector, or a plugged flue pushes flame out the front. Pull the inducer/collector look, confirm the trap is flowing, and check PVC/vent. Filter, heat speed, dirty A-coil, rollout reset.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on Carrier 58SU0 is a CO / fire call — cracked primary, misaligned burners, or a failed secondary. Confirm CO in the space before you leave.

   Expect: One clean cycle after the cause is fixed. No second reset as a repair.

Hardened 34.

1. 1 Watch one full try — do not reset

   Write the Carrier 58SU0 code. Watch inducer, HSI glow, valve click, flame, µA. The step that dies is the repair. Resetting only burns another try into a harder lockout. HSI, gas, rod, ground. Do not sit on resets.

   Expect: W → No PVC → 58CTA cousin

2. 2 Gas and the valve

   Cock open, inlet pressure, then manifold on the stage that is failing vs the rating plate. 24 VAC at the valve during trial? Coil ohms? A valve that never opens looks like a ‘bad igniter’ if you did not watch the sequence.

   Expect: Inlet and manifold on the plate. 24 VAC only during trial.

3. 3 Flame prove, then the lockout timer

   Glow + valve + no prove → rod, ground, or IFC sense. Prove then drop → µA / PS / limit. Soft lockouts on this family often auto-reset on a long clock — do not sit on resets.

   Expect: Stable µA after light-off. No new PS/limit code.

Valve relay fault, or missing/wrong model plug on boards that use one.

1. 1 Read the plug / card on THIS cabinet

   On Carrier 58SU0 the model plug or shared-data card must match this cabinet size. A 100 kBtu plug in an 80 cabinet overfires and limits. A leftover memory card after a control swap throws d0/d4-class codes. Most Comfort 80 STA boards are not TP6-style plug cabinets — check the door before you hunt a plug.

   Expect: Plug / card part matches the rating plate and the replacement control bulletin.

2. 2 Reseat, then confirm the menu

   Kill power, reseat, restore power. On communicating plates open the setup and confirm unit size / motor HP. Valve harness and 24 VAC during trial.

   Expect: IFC recognizes the plug. No size-mismatch code.

3. 3 Do not run it mismatched

   A wrong plug is not a ‘temporary’ fix. It will throw limit, ignition, or blower-band codes that look like bad parts. Fit the correct plug, then re-run the original complaint.

   Expect: One clean sequence on the correct identity.

Flame sense while GV de-energized.

1. 1 Shut the gas cock first

   On Carrier 58SU0 the rod sees flame with the valve de-energized. Shut the cock. If flame stays, the valve is leaking — replace it. Do not jump or energize it as a test. Leaking valve. Shut the cock.

   Expect: Flame dies when the cock is shut.

   If fail: Leaking valve. Replace. Do not leave the unit.

2. 2 Rod and IFC flame circuit

   If flame dies with the cock shut, the rod is shorted to ground or the IFC flame circuit is lying. Unplug the rod, inspect ceramic, and check the harness for a rub-through on the collector.

   Expect: Rod isolated from ground. IFC does not report flame with the rod unplugged.

3. 3 Confirm valve solenoids are actually off

   Measure 24 VAC at both solenoids after the call drops. A welded IFC relay or a shorted W will keep a solenoid wet and look like ‘unexpected flame.’ Replace the IFC only after the valve and rod are proven.

   Expect: 0 VAC at both solenoids after the call. No flame.

Draft switch still made when it should be open.

1. 1 Unplug the switch with the inducer off

   On Carrier 58SU0 this code is the switch made when the inducer is not running. Unplug that switch only. This is Category I metal vent. There is no 96% collector or PVC trap to prime.

   Expect: Code changes when the harness is open.

   If fail: Wrong switch, or the IFC input is shorted.

   If pass: Switch/hose path. Go to step 2.

2. 2 Water in the hose vs a welded switch

   Blow the hose. Water holding residual draft is more common than a welded switch. Confirm the hose is not teed and not routed uphill. Ohm the switch with the hose off. Cap, liner, common-vent tables, inducer wheel, hose on the inducer housing.

   Expect: Hose empty. Switch opens with no draft.

3. 3 IFC input last

   If the switch ohms open, the hose is dry, and the code does not change with the harness off, the Carrier 58SU0 IFC input is shorted. Do not keep swapping switches. Welded switch, residual chimney draft, inducer still spinning.

   Expect: Known-good open switch + open harness = code gone. If not, IFC.

24 V fuse on the IFC.

1. 1 Find the 24 V short before you replace the fuse

   On Carrier 58SU0 an open IFC fuse is a short on W, Y, G, HUM, EAC, or a data pair landed on 24 V. Unplug the thermostat / data harness first. Tstat short, HUM/EAC, valve harness. Replace fuse only after the short is gone.

   Expect: Fuse out. 24 V transformer still good. Harness isolated.

2. 2 Replace the fuse with the harness off

   New fuse, harness still off. If it holds, the short is in the field wiring or accessory. Plug one circuit back at a time (W, then Y, then HUM/EAC).

   Expect: Fuse holds with harness off. Dies when the shorted lead lands.

3. 3 If it blows with the harness off

   Short is on the Carrier 58SU0 IFC or the gas-valve harness. Unplug the valve and try once more. Repeat blow with valve unplugged = IFC.

   Expect: Do not keep feeding fuses into a welded valve coil or a burned IFC trace.

Category I draft will not prove. NEVER a PVC condensate trap.

1. 1 Prove inducer and the live code

   Do not pull power — that clears the Carrier 58SU0 active code. Write 31. Is the inducer spinning on this call? Silent inducer is not a pressure switch. This is Category I metal vent. There is no 96% collector or PVC trap to prime.

   Expect: 120 VAC at the inducer on a heat call. Wheel turning.

   If fail: 120 VAC missing → IFC / door switch / incoming power. Voltage + dead motor → inducer.

   If pass: Inducer running + this code → trap, hose, switch, vent (step 2).

2. 2 Trap, collector, and the hose

   A drowned 96% trap will not make collector pressure. Pull the trap, flush it, confirm the collector box and inducer outlet are not full. Hose cracked, off the port, or full of water fakes an open switch. Cap, liner, common-vent tables, inducer wheel, hose on the inducer housing.

   Expect: Trap flowing, hose dry and downhill, ports not swapped on a two-switch board.

3. 3 Switch ohms, then vent load

   Ohm the pressure switch: NO, must close when that inducer speed is making the collector setpoint. If the switch is good, walk the PVC — bird screen, ice, long 2" run, wind, shared chase. Chimney liner, cap, masonry debris, wind.

   Expect: Switch closes on known draft. Vent unrestricted.

   If fail: Replace the open switch only after the vent and trap are proven.

   If pass: Switch closes on draft and the code stays → IFC input.

Airflow or flame-containment. Dirty A-coil on an 80% is the winter/summer crossover call.

1. 1 Treat this as flame outside the exchanger

   Do not reset the Carrier 58SU0 rollout and walk away. Write the code, kill the call, and look for flame at the burners, crossover, and vestibule before you do anything else.

   Expect: No flame outside the exchanger. Burners seated. Crossover lit.

   If pass: Still inspect secondary / flue / condensate before you reset.

2. 2 Secondary, condensate, and flue

   Blocked secondary, drowned collector, or a plugged flue pushes flame out the front. Pull the inducer/collector look, confirm the trap is flowing, and check PVC/vent. Filter, blower speed, A-coil on top of the furnace — 80% coils load up and limit. Rise vs plate. Rollout: exchanger / burners / flue. Manual reset.

   Expect: Dry collector, open flue, trap clear.

3. 3 Reset only after the cause is fixed

   Manual-reset rollout. Recurring rollout on Carrier 58SU0 is a CO / fire call — cracked primary, misaligned burners, or a failed secondary. Confirm CO in the space before you leave. Held open → 13.

   Expect: One clean cycle after the cause is fixed. No second reset as a repair.

Trial without flame sense. Repeats → 14.

1. 1 Measure µA while it is running

   Do not clean first and guess. Series-µA the rod on Carrier 58SU0 during a call. Falling µA is oxide, ground, or a dying flame. A hard drop is often the pressure switch or limit opening, not the rod. HSI, gas cock, manifold, rod, ground.

   Expect: Typically ≥ 1.0 µA DC on this family — confirm the door card. Rising while running.

2. 2 Rod, ground, then gas

   Clean the rod with a non-scratch pad — not sandpaper. Chassis ground to burner box and IFC (painted screw and missing star washer are classic). Then manifold on the stage that is dropping and confirm LP conversion leftovers. Fix 31 metal-vent draft before a fair 34 test.

   Expect: Manifold on the rating plate. Solid burner-box ground.

3. 3 Prove it is not a switch opening

   Watch the live Carrier 58SU0 display when flame drops. A PS or limit code that appears at the drop means the rod was a victim. Inducer, hose, vent wind, or a limit on a wrong heat tap.

   Expect: µA stable and the live code stays a flame code — then rod/valve/IFC. Otherwise chase the new code.