Carrier furnace Status code 12: Blower running after power up

Code 12 means the blower is running for 90 seconds because power was restored during a call for heat, or the call ended during the blower on-delay. It is normal behaviour, not a fault.

Applies to: Carrier 58MCB four-way multipoise condensing gas furnaces and related models using the same integrated furnace control. Bryant and Payne furnaces of the same generation use the same control and code set.

What it means

This code documents a deliberate safety behaviour rather than a problem. If the furnace is powered up while the thermostat is calling for heat, or if the call ends during the blower's on-delay period, the control runs the blower for 90 seconds. The reason is that the control cannot know what state the heat exchanger was left in — there may be residual heat with no air moving over it — so it clears the situation before doing anything else. Seeing this code after a power cut, a breaker reset, or a service visit is expected. It is worth recognising precisely so it is not mistaken for a fault.

Most common causes

• Power was restored while the thermostat was calling for heat, after a power cut or a breaker reset.
• The furnace was switched back on at its service switch during a call for heat.
• The thermostat call ended during the blower on-delay period.
• A service visit where power was cycled with the thermostat calling.
• A brief power interruption during a heating cycle.

How to diagnose it

1. Note whether power was recently interrupted. That explains the code completely.
2. Note whether the blower stops after about 90 seconds. If it does, the furnace has done what this code describes and there is nothing wrong.
3. If the blower runs continuously rather than for 90 seconds, that is a different situation and the code is not the explanation.
4. Check whether the furnace then heats normally on the next call.

How to fix it

1. Nothing to fix. Let the 90 seconds elapse and the furnace will resume normal operation.
2. If frequent power interruptions are producing this repeatedly, the electrical supply is worth investigating — not because of this code, but because interruptions are hard on the equipment.
3. If the furnace does not heat normally after the blower stops, read the display again for a different code.

Notes

Codes that describe normal behaviour are worth knowing, because a furnace displaying something after an unrelated power cut naturally looks broken. This one means the control is clearing the heat exchanger before restarting, which is what it should do.

Related

• Status code 11 — No previous code stored
• Status code 33 — Limit circuit fault

All codes for this manufacturer

Carrier, Bryant and Payne furnace status codes — every code, with what differs between models.

Sources

• Carrier 58MCB Service Manual, 327884-101 Rev. B — status codes