Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
ICP Furnace E4 Error Code — What It Means
E4 on an ICP furnace (International Comfort Products — the parent platform for Heil, Tempstar, Comfortmaker, and Day & Night) indicates an open limit circuit. The high-temperature limit switch or rollout switch has opened, cutting power to the gas valve and stopping heating. Unlike E1 (pressure switch), E4 points specifically to the limit/rollout safety chain. The furnace will not restart until the limit device resets and the E4 is cleared.
Common Causes
- Overheating heat exchanger — Restricted airflow from a clogged filter, blocked registers, or low blower speed allows heat exchanger temperature to exceed the limit’s trip point.
- Rollout switch activated — Flames rolling out of the burner compartment (due to blocked flue, cracked heat exchanger, or incorrect gas pressure) trip the rollout limit, which requires manual reset.
- Failed blower motor — If the blower does not start or runs slowly, heat accumulates and the limit opens even with a clean filter.
- Incorrect gas pressure — High gas pressure causes large, rolling flames that can activate the rollout switch even without a mechanical blockage.
Step-by-Step Fix
- Replace the air filter and check registers — Start with the simplest cause. A clogged filter is responsible for the majority of E4 codes.
- Locate and reset the limit switches — The high-limit switch is on the plenum above the heat exchanger; the rollout switch is near the burner compartment. Press the manual reset button on any tripped switch. If the rollout switch has tripped, investigate the cause before resetting — rollout indicates a combustion safety problem.
- Check blower operation — Initiate a heat call and confirm the blower starts within 30–60 seconds. If the blower does not start, test the blower motor and its run capacitor.
- Inspect burner flames — If safe to observe (no gas odor, no CO alarm), watch the burner flames during a heat call. Flames should be steady and blue. Yellow, wavering, or rolling flames indicate incomplete combustion or a cracked heat exchanger.
- Clear the E4 code — Power cycle the furnace (30 seconds off at the disconnect) and run a complete heat cycle. If E4 returns within the same cycle, the root cause is not resolved.
Parts Often Needed
| Part | Notes |
|---|---|
| Air filter | Always first check on E4 |
| High-limit switch | Auto or manual reset; verify trip temperature matches OEM spec |
| Rollout limit switch | Manual reset only; replace if reset button does not hold |
| Blower run capacitor | Inexpensive first test before replacing the blower motor |
Need Replacement Parts?
Before ordering, match the furnace part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
A tripped rollout switch indicates flames are escaping the burner box — potentially caused by a cracked heat exchanger or blocked flue. Do not operate the furnace after a rollout trip until a licensed technician inspects the heat exchanger and venting for safety.