Furnace Limit Switch Cross-Reference & Fitment Guide

What a limit switch actually is

A furnace limit switch is a thermostat. That is the whole device. It is a temperature-operated switch, normally closed, wired in series with the gas valve circuit, and it opens when the air around its sensing element gets hotter than its rating. When it opens, the furnace stops making heat.

It is not electronic. It does not talk to the control board beyond being open or closed. It has no settings on most furnaces, no polarity, and nothing to program. People treat it as a mysterious safety component because it lives inside a furnace, but the thing in your hand is a bimetal disc or a bimetal stem that bends at a set temperature and snaps a contact open.

That is why universal replacements work, and it is why we sell far more universals than OEM part numbers across this counter. If the cut-out temperature is close, it opens the right circuit, and it physically fits, it will do the job the original did.

Everything below is the detail behind those three tests.

The three limits people all call “the limit”

Three different parts get called “the limit switch,” and they are not interchangeable with each other even though they look similar. Goodman’s service literature names them plainly:

Source: Goodman GMVM97/GCVM97 Service Manual, p.55. Goodman also puts a boxed warning on the auxiliary: “To avoid possible fire, only reset the auxiliary limit control once. If it should open a second time, a qualified servicer must determine why the auxiliary limit opened before resetting again.”

The pattern is consistent across brands: reset type is assigned by what the switch is protecting. Automatic where the fault is expected to clear on its own, manual where a human needs to look at the furnace before it runs again. Hold on to that — it is the one rule on this page we will not bend.

Telling the types apart by sight

Half of the substitution test is physical, and a table cannot show you a shape. These are photographs of parts on our own shelf. Tap any of them for the full specification.

Body style and reset type are separate questions — the same shape exists in both. Read the L number and check for a button before you match anything.

What happens when one opens

The sequence is close to identical across manufacturers. The gas valve drops immediately, the blower is force-run to pull the leftover heat out of the heat exchanger, and the board waits for the switch to re-close. If it does not re-close fast enough, or if it trips too many times, the board escalates to a lockout.

Nordyne states it in one sentence:

“The main limit control is an automatic reset type. It reacts to abnormally high air temperatures in the heat exchanger area. If the main limit opens, the gas valve is de-energized and the induced draft and main blower motors continue to run. The main limit will automatically reset after the temperature is reduced.”

The published timings are worth knowing, because they explain symptoms people describe at the counter:

Lennox is the odd one out — it shuts the inducer off where everyone else runs it. Worth knowing before you diagnose a Lennox by ear.

Two symptoms fall straight out of that table. “The blower runs forever and there is no heat” is the limit circuit open, on almost any brand. And “it heats for a while, quits, then comes back an hour later” is an automatic-reset primary limit tripping and re-closing, which is a signal about airflow, not a signal that the switch is bad.

The three things that have to match

This is the whole substitution test, in the order we check it at the counter.

The differential is the fourth thing, and it matters less than the other three but it is worth understanding. Every automatic-reset limit has a cut-out and a cut-in, and the gap between them is the differential. An L170-40 opens at 170°F and re-closes at 130°F. An L170-20 opens at the same 170°F but re-closes at 150°F. Same cut-out, different recovery behaviour. If a furnace short-cycles after a limit change, a differential mismatch is a likely reason.

The one substitution we refuse

Reset type is not a preference. The furnace manufacturer assigns it by function, and swapping it does two separate kinds of damage.

Auto fitted where manual belongs is the dangerous direction. A manual-reset limit sits in the positions where the manufacturer requires a human to inspect the appliance before it runs again — rollout, blower compartment, blocked vent. Put an automatic part there and the condition that was supposed to force a service call now silently clears itself and the furnace re-fires. Flame rolling out of the burner box becomes a thing that happens quietly, over and over, with nobody ever seeing it.

There is a second consequence that is less obvious. Several control boards identify which safety opened by how long it stays open. York, in its own words: “If, after fifteen minutes, the main limit still has not closed, the control will assume that a manual-reset rollout switch has opened and will begin to flash the 5 Red Flash error code.” Substitute an auto-reset part and the board never reaches that conclusion. The rollout code is never reported and the next technician is sent down the wrong path.

Manual fitted where auto belongs is the nuisance direction, and it is still wrong. The primary limit is designed to open and re-close on its own; that is why the manufacturer publishes a cut-in temperature for it. Put a manual-reset part in that position and every dirty-filter trip becomes a no-heat call. Worse, the boards read a limit that cannot re-close as a dead blower — Rheem declares exactly that after 150 seconds — and the next person replaces a perfectly good blower motor.

Every manufacturer prints the same boilerplate on safety controls: replace only with identical OEM parts, and never bypass or jumper a safety switch. We stock and sell universals daily and they work. Reset type is where the boilerplate and the counter agree.

Supco SHL501–SHL525 stem limits

These are the universal stem-type limits that replace most board-mounted OEM limits on gas furnaces. Supco catalogs them as “Gas Furnace Limits and Plenum Thermostats,” style 36TX01B, 350°F maximum ambient. All are SPST and open on rise. Supco publishes a cut-in temperature and a differential for every one, which is what makes them automatic reset — Supco’s manual-reset limits are the separate SHM series.

Read this table for the traps, not just the temperature

Electrical rating for the whole SHL5xx family is Supco’s 36T class: 120 VAC 15 A resistive, 3 A motor FLA, 12 A LRA, 125 VA pilot duty; 240 VAC 10 A resistive, 1.5 A FLA, 6 A LRA, 125 VA pilot duty. Range −40°F to 350°F.

What Supco does not publish for this family: mounting hole spacing or hole diameter (only the board footprint), terminal type and size, and the words “automatic reset” (the cut-in temperature and differential are published instead, which tells you the same thing). We are not going to invent those numbers. Bring the switch in and we will compare it physically.

Supco SHL120–SHL250 snap-disc limits

A different family under the same SHL prefix, and the source of a lot of mis-ordering. These are snap-disc limit controls, style 60T11, supplied with an adapter flange plate. They are not stem type and they are not interchangeable with the SHL5xx above.

Rating class here is 60T, not 36T — a heavier switch. 120 VAC 25 A resistive, 14 A FLA, 72 A LRA; 240 VAC 25 A resistive, 10 A FLA, 60 A LRA; 125 VA pilot duty. Supco’s matching manual-reset discs are the SHM series (SHM125 through SHM350, style 60T15), and its close-on-rise fan controls are the SHF series (style 60T12). Three families, three jobs, one shelf — read the letter after SH.

SHL210, SHL300 and SHL325 appear in an older Supco catalog but not in the current full-line book. If you need one of those, ask — current availability needs checking rather than assuming.

OEM number to universal cross-reference

These crosses are Supco’s own, published in its full-line catalog. Where a manufacturer has not published a cross, we have left it out rather than guessing.

If your OEM number is not on this list, that does not mean nothing replaces it. It means Supco has not published a cross for it. The three tests still apply — read the L number off the old switch, note whether it is auto or manual reset, measure the stem, and we will match it from the tables above.

Adjustable limits (AT series)

Supco’s AT series are dial-adjustable open-on-rise thermostats with a 40° adjustable range and quarter-inch quick-connect terminals. They are useful when you need a temperature that no fixed disc covers, or on equipment where the original is long gone.

Supco publishes the range, the differential and the 60T electrical rating for these, and nothing else — no stem length, no mount pattern, no pole configuration. The disc diameter and “SPST” descriptor you will find on retail listings come from distributors, not from Supco, so we do not print them here.

Fan/limit controls (5D51)

A fan/limit control is two switches on one bimetal element in one housing: a close-on-rise fan switch that starts and stops the blower, plus an open-on-rise limit switch that is the safety. White-Rodgers describes the 5D51 as regulating “fan or blower operation” while acting “as high limit safety control.”

These turn up on older furnaces and on any unit without a board doing the blower timing. On a modern board-controlled furnace the board handles the blower and only a plain limit remains — which is why a 5D51 will not substitute for an SHL, or the reverse.

Retail listings for the 5D51-35 give insertion lengths of 0.5″, 4-3/8″ and 5″ depending on where you look. White-Rodgers publishes 5″ element length. We use the manufacturer’s figure.

Fault codes for an open limit, by brand

Every brand has shipped several generations of control board and the code changes between them. There is no such thing as “the Goodman limit code.” Each entry below is tied to a specific board generation, because publishing a brand-level code without that is how a parts page gets a reputation for being wrong.

Two brands do not use a simple flash count and are routinely miscounted:

York’s four-flash code on some models also covers a defective temperature sensor and an open 24-volt fuse, not the limit alone. Lennox’s communicating iComfort systems use a separate numbered alert scheme that we have not verified a limit code from, so we are not printing one. And a limit code always means the circuit is open — not that the switch has failed. Check the wiring and the connectors before you buy anything.

When the limit is not the problem

This is the part worth reading twice. A limit switch that opens is usually doing its job correctly, and the fault is upstream of it. Every manufacturer’s own troubleshooting text says the same thing in the same order.

Goodman lists the probable causes of its four-flash code as “low conditioned air flow due to dirty filter or resistance in duct work, faulty limit, faulty blower, or blower speed set to low.” Note where “faulty limit” sits in that list — third. York’s list is “a dirty filter, improperly sized duct system, incorrect blower speed setting, incorrect firing rate, loose limit switch wiring or faulty blower motor.” ICP adds “loose blower wheel” and “improper limit switch or no limit gasket.”

In plain terms, before you replace the switch:

If you are on your second or third limit switch in as many seasons, the switch is not the problem. Something is making that furnace run hotter than it was built to run, and the limit is the smoke alarm, not the fire.

Questions we get at the counter

Sources