Lennox SLP98UHV Error Code E 403: LSOM - Compressor Short-Cycling
What Does Code E 403 Mean?
On the Lennox SLP98UHV, E 403 is a Lennox System Operations Monitor (LSOM) code reporting that the outdoor air-conditioning compressor is short-cycling — starting and stopping in rapid succession, running less than 4 minutes at a time — after an outdoor-unit pressure trip. This is a cooling-side condition monitored by the communicating system, not a furnace combustion fault.
Short-cycling is hard on the compressor: every start draws a large surge of current, and frequent starts without enough run time cause heat buildup and premature wear. E 403 sits among the LSOM compressor codes. It is related to E 401 (the compressor ran more than 18 hours, unable to satisfy the call) and E 402 (an outdoor-unit pressure trip), and it should not be confused with the harder mechanical and electrical faults nearby such as E 404 (compressor rotor locked) or E 405 (compressor open circuit), which are not homeowner-fixable.
The most common homeowner-addressable cause is a dirty outdoor condenser coil or debris blocking airflow around the unit. When grass clippings, cottonwood seeds, leaves, or dirt clog the coil, the outdoor unit cannot reject heat, refrigerant head pressure climbs, and the high-pressure safety trips — driving the short-cycling. Other causes — a failing outdoor fan motor, incorrect refrigerant charge, or electrical faults — are sealed-system and electrical work that must go to a licensed technician.
What You'll Notice
- The outdoor AC unit starts and then shuts off again after only a minute or two
- The home is not cooling well or the AC seems to constantly cycle
- You hear the outdoor unit clicking on and off repeatedly
- The 7-segment display shows E 403 when you recall codes
- The outdoor condenser coil is matted with grass, leaves, seeds, or dirt
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty outdoor condenser coil causing high head pressure | Most common | ✓ DIY fix → |
How This Is Diagnosed
The homeowner-safe cause is checked first: whether airflow around the outdoor unit and through the condenser coil is blocked. If the coil and surroundings are clean and E 403 continues, the cause is in the sealed refrigerant system, the outdoor fan motor, or the electrical circuit, all of which a licensed technician diagnoses with gauges and meters.
How to Fix It: Clear Debris and Gently Rinse the Outdoor Condenser Coil
What You'll Need
Steps
- Turn off electrical power at the breaker, shut off the gas supply valve, and pull the outdoor-unit disconnect Switch the furnace breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe). Then turn off power to the outdoor unit at its disconnect box, usually mounted on the wall beside the condenser. If you smell gas, leave immediately and call your gas company.
- Clear vegetation and debris around the outdoor unit Pull away leaves, grass clippings, weeds, branches, and any other debris from around and against the outdoor condenser. Aim for at least 2 feet of open clearance on every side so the unit can pull in and exhaust air freely.
- Gently rinse the condenser coil from the inside out Using a garden hose at low pressure — never a pressure washer — direct water through the coil from the inside out to flush dirt back out the way it came in. Keep the stream gentle and straight through the fins. Do not bend the delicate aluminum fins, and do not use any chemical or acidic coil cleaners.
- Visually check the outdoor fan (do not service it) Look at the fan on top of the unit for cracked blades or debris jamming it. If the blades are damaged, the motor will not turn freely, or it makes grinding noises, stop and have a technician handle the fan motor — this is not a homeowner repair.
- Restore power and gas, then test Let the coil dry, then restore the outdoor disconnect, turn the gas valve to ON (handle parallel to the pipe), and switch the furnace breaker to ON. Set the thermostat to call for cooling and confirm the compressor now runs continuously for at least 4 minutes.
When to Call a Professional
Contact a licensed HVAC technician if:
- E 403 returns after clearing debris and rinsing the condenser coil
- The outdoor fan motor does not spin or makes grinding noises
- The compressor makes unusual clicking, buzzing, or humming sounds
- The system was recently serviced and refrigerant may have been added or removed
- E 404 (compressor rotor locked) or E 405 (compressor open circuit) appears alongside E 403
Frequently Asked Questions
Why is short-cycling bad if the AC still turns on?
Each compressor start draws a large current surge, and starting and stopping every couple of minutes builds up heat and wears the motor and electrical parts. Left unaddressed, short-cycling can lead to premature compressor failure, so it is worth stopping quickly.
Can I use a coil cleaner spray or a pressure washer to clean the coil faster?
No. Chemical coil cleaners can damage the coil and fins, and a pressure washer will flatten the thin aluminum fins and cut airflow permanently. Stick to a garden hose at low pressure, rinsing gently from the inside out.
The coil looked clean but E 403 keeps coming back — now what?
If airflow around a clean coil is not the problem, the cause is likely a failing outdoor fan motor, an incorrect refrigerant charge, or an electrical fault. Those are sealed-system and electrical repairs that a licensed HVAC technician needs to handle.
✓ Verified against manufacturer service manual — March 2026