Furnace overheats and trips the high limit
The furnace lights normally, runs for several minutes, then shuts the burners down while the blower keeps going. It is overheating, and the cause is almost never the limit switch itself.
What is actually happening
The high limit is a normally-closed temperature switch in the heat exchanger compartment that opens when the air leaving the furnace exceeds a set temperature. Opening it cuts the gas valve while leaving the blower running to carry the heat away. A furnace that keeps tripping on limit is a furnace that is producing more heat than the airflow can remove.
What causes it
Nearly every cause is a lack of airflow: a clogged air filter, closed or blocked supply registers, blocked return grilles, an undersized or crushed return duct, a blower wheel packed with dirt, or a blower motor running slow or not at all. Several manufacturers publish a separate code specifically for a limit that opened because the blower failed, which is the same overheat with the cause already identified. Less common but real: an oversized furnace for the duct system, or a gas pressure set too high.
The codes that report this
11 codes across 9 control families. Each links to what that manufacturer says about it.
| Brand | Code | What it reports |
|---|---|---|
| Armstrong | 7 flashes | Primary or secondary limit open, or Watchguard after three minutes open |
| Carrier | Status code 13 | Limit circuit lockout |
| Goodman | 4 flashes | High limit switch is open — the furnace is overheating |
| Goodman | E3 | High limit switch is open — the furnace is overheating |
| Goodman | 11 red flashes | Limit switch open due to blower failure |
| Heil | 6 + 4 flashes | Soft lockout — limit circuit opened during the run |
| Heil | 7 flashes | Soft lockout — limit trips taking longer than two minutes to reset |
| Lennox | Alert code 251 | Primary limit switch open |
| Lennox | Alert code 274 | Primary limit switch open more than three minutes — Watchguard |
| Rheem | 3 blinks | Limit switch open |
| York | 11 red flashes | Limit switch open more than five minutes — blower failure |
Worth knowing
Replacing a limit switch that is doing its job leaves the overheat in place and removes the protection against it. Repeated limit trips stress the heat exchanger, and the heat exchanger is the component whose failure puts combustion gases into the supply air. Find the airflow restriction.
Other symptoms
- Lights, then shuts off after a few seconds
- Igniter glows but the burners never light
- Nothing at all, no lights on the control board
- Inducer runs but the burners never come on
- Runs briefly, stops, starts again, then quits
Sources
- Allied Air Enterprises — A80UH1E and 80G1UHE Installation Instructions 507330-03, Table 7: Diagnostic Codes for -02 Control
- Carrier 58MCB Service Manual, 327884-101 Rev. B — status codes
- Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)
- International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)
- Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting
- Rheem Professional Technical Support — Standard Gas Furnace Diagnostic Flash Codes
- Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, fault code tables and retrieval procedure