Lennox SLP98UHV Error Code E 403: LSOM - Compressor Short-Cycling
What Does Code E 403 Mean?
Error code E403 on the Lennox SLP98UHV indicates the outdoor AC compressor is short-cycling — turning on and off in rapid succession, running less than 4 minutes per cycle. This is triggered by a high-pressure trip on the outdoor unit.
Short-cycling is harmful to the compressor because it causes excessive wear on the motor and electrical components. Each time the compressor starts, it draws a high surge of current, and frequent starts without adequate run time can lead to premature compressor failure.
The most common cause is a dirty outdoor condenser coil. When the coil is clogged with dirt, grass clippings, cottonwood seeds, or other debris, the outdoor unit cannot reject heat efficiently. This causes the refrigerant pressure to build up until the high-pressure safety switch trips. Other causes include a failing outdoor fan motor, incorrect refrigerant charge, or a restricted airflow path around the outdoor unit.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty outdoor condenser coil causing high head pressure | Most common | ✓ DIY fix → |
How to Fix It: Clean the Outdoor Condenser Coil
What You'll Need
- Garden hose with spray nozzle
- Coil cleaner spray (optional, available at hardware stores)
- Flashlight
Steps
- Turn off electrical power and gas supply Turn off the furnace at the indoor circuit breaker. Turn off the outdoor AC unit at its disconnect box (usually mounted on the wall near the outdoor unit). Turn the gas shutoff valve to the OFF position at the furnace.
- Clear debris from around the outdoor unit Remove any leaves, grass clippings, branches, or other debris from around the outdoor condenser unit. Maintain at least 2 feet of clearance on all sides for proper airflow.
- Clean the condenser coil Using a garden hose (not a pressure washer, which can damage the fins), spray water from the inside out through the condenser coil to flush out dirt and debris. If the coil is heavily soiled, apply a commercial coil cleaner spray first, let it sit per the manufacturer's directions, then rinse thoroughly. Be gentle — the aluminum fins are delicate.
- Inspect the outdoor fan Look at the fan on top of the outdoor unit. If the blades are cracked, the motor is not spinning freely, or the motor makes unusual noises, a technician will need to replace it.
- Restore power and test Turn on the outdoor disconnect and indoor circuit breaker. Turn the gas supply valve back to ON. Set your thermostat to call for cooling and let the system run. The compressor should run for at least 4 minutes continuously.
When to Call a Professional
Contact a licensed HVAC technician if:
- E403 returns after cleaning the condenser coil
- The outdoor fan motor is not spinning or makes grinding noises
- The compressor makes unusual clicking or buzzing sounds
- The system was recently serviced and refrigerant may have been added or removed
- You also see E404 (compressor rotor locked) or E405 (compressor open circuit)
✓ Verified against manufacturer service manual — March 2026