FieldBench
Carrier codes

Carrier

21

Lockout

Gas-heat lockout / model plug

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

Walk 21 in Diagnose

7 plates — walk the one on the cabinet

First check

1 Read the plug / card on THIS cabinet

On Carrier 58SP0 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.

The rest of the walkthrough is Diagnose.

On the plates

  • Model plug seated for THIS 59TP6 cabinet. Wrong plug = wrong CFM and false 41/44.
  • Miswired valve.
  • Valve harness and 24 VAC during trial.
  • If this cabinet has a model plug, it must match THIS Comfort 96 cabinet — do not steal a 59TP6 plug and expect correct CFM.
  • Most Comfort 80 STA boards are not TP6-style plug cabinets — check the door before you hunt a plug.

Plates that use 21

Same number, other makers

21 on Carrier is not the same code on these plates.