Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Tempstar Furnace E4 Error Code — What It Means
E4 on a Tempstar furnace signals an open limit circuit — the high-limit switch or rollout switch has tripped due to excessive heat. Tempstar is an ICP brand (same parent as Heil, Comfortmaker, and Carrier) and the control board logic is shared across these families. When the limit circuit opens, the board shuts off the gas valve immediately and flashes E4. The blower typically keeps running to cool the heat exchanger. Repeated E4 faults point to an airflow problem or a failing heat exchanger.
Common Causes
- Restricted airflow from a dirty filter — The single most common cause. A clogged air filter starves the heat exchanger of cooling air, causing temperatures to exceed the limit set-point within minutes.
- Blower motor or capacitor failure — A blower running below rated speed — or not at all — causes heat to build rapidly. A bad run capacitor is the most frequent blower-related culprit.
- Rollout limit tripped — Rollout switches mount on the burner assembly and trip if flames extend outside the combustion chamber. A cracked heat exchanger or incorrect gas pressure can cause rollout.
- Blocked flue or exhaust restriction — A partially blocked flue increases back-pressure in the combustion chamber, causing heat and combustion gases to exceed normal limits.
Step-by-Step Fix
- Replace the air filter — Check and replace the filter first. Many E4 faults resolve with a fresh filter alone.
- Confirm blower starts and runs at full speed — Switch the thermostat to fan-only and verify the blower runs strongly. If it hums or starts slowly, test the run capacitor with a capacitor meter and replace if out of spec.
- Inspect the rollout switch — The rollout switch is located near the burners and usually has a manual-reset button. If tripped, press to reset. Investigate the root cause before restarting — do not simply reset and ignore repeated rollout trips.
- Check the high-limit switch continuity — With power off, test the limit switch with a multimeter. An open reading at room temperature means the switch has failed; replace it with the matching temperature-rated switch.
- Inspect the flue pipe — Visually inspect both indoor and outdoor sections of the exhaust pipe for blockage, sagging that traps condensate, or disconnected joints.
- Reset and test — Power cycle, restore heat call, and confirm the furnace completes a full heating cycle without re-triggering E4.
Parts Often Needed
| Part | Notes |
|---|---|
| High-limit switch | Match the °F trip rating and mounting configuration |
| Rollout limit switch | Many are auto-reset; persistent trips require root-cause repair |
| Blower run capacitor | Match µF and VAC rating exactly |
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
If the rollout switch trips repeatedly after resetting, have a technician check for a cracked heat exchanger before using the furnace — cracked exchangers are a carbon monoxide hazard and require heat exchanger or furnace replacement.