W4
Universal HVAC System
Severity: ModerateWhat Does This Error Mean?
The W4 error means the condensate pump that removes water from the drain pan has stopped working. Without the pump, water builds up and the system shuts down to prevent flooding. The pump usually needs cleaning or replacement.
Affected Models
- Basement or attic air handlers without gravity drain
- Mini-split systems with condensate pumps
- Central HVAC systems in low-clearance installations
Common Causes
- Condensate pump motor has burned out or seized
- Pump reservoir is clogged with algae or slime and cannot intake water
- Pump float switch is stuck and falsely signals that the reservoir is full
- Power supply to the pump is interrupted — loose wiring or a tripped outlet
- Pump discharge line is blocked, creating back-pressure that stops the pump
How to Fix It
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Turn off the HVAC system at the thermostat.
Stop the system from producing more condensate water while you diagnose the pump.
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Find the condensate pump — a small white or grey box near the indoor air handler with a reservoir tank on top.
Check that it is plugged into a working outlet. Try plugging a lamp into the same outlet to confirm the outlet has power.
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Remove the pump reservoir lid and clean out any algae, slime, or debris inside.
Use a soft brush and diluted white vinegar. Rinse thoroughly with clean water before reassembling.
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Manually pour a small amount of water into the pump reservoir to test if the pump activates.
You should hear the pump motor start when water reaches a certain level. If it stays silent, the pump motor has likely failed.
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If the pump does not respond, replace the condensate pump unit.
Condensate pumps typically cost $30–$80 and are straightforward to replace. An HVAC technician can do this on a standard service call.
When to Call a Professional
If cleaning the pump does not fix the W4 error, the pump needs replacing. An HVAC technician can confirm the pump is the problem and install the correct replacement.
Frequently Asked Questions
Do all HVAC systems have a condensate pump?
No — only systems where the drain pan cannot drain by gravity. If your air handler is in a basement below the drain outlet, a pump is needed to push water upward.
How long does a condensate pump last?
Most condensate pumps last 5–10 years with proper maintenance. Cleaning the reservoir every year extends the life significantly.
Can I replace a condensate pump myself?
Yes — it is one of the simpler HVAC repairs for a handy homeowner. Disconnect the inlet tube, drain line, power cable, and float switch wire. Install the new unit in reverse order.