Water on the passenger floor when running AC is almost always a clogged evaporator drain. The condensate has nowhere to go and overflows into the cabin. Easy fix if caught early.
Most common AC leak by far. The drain at the bottom of the HVAC box plugs with leaves, dust, and bio-gunk. Condensate backs up and overflows into the passenger footwell.
The rubber drain hose under the car has fallen off its barb. Water still drains out but lands inside the firewall area and seeps into the cabin.
The plastic HVAC box has cracked from age or from a hard hit during a repair. Water seeps where the case meets the floor.
Coolant in the carpet, not just water. Sweet smell and may fog the windshield. Easy to distinguish from condensate by smell and stickiness.
Not AC related but often blamed on AC. Heavy rain causes water on the passenger floor. Test by running a hose on the windshield and looking for entry points.
Front sunroof drains run down the A-pillars and exit near the firewall. Clogged drains overflow into the cabin. Looks identical to AC overflow.
Leaves and debris in the cowl panel (at the base of the windshield) clog the drain. Water can enter the cabin through the firewall.
| Likely Cause | Typical Cost | DIY Difficulty | Severity | Likelihood |
|---|---|---|---|---|
| Clogged Evaporator Drain Tube | $0-$30 unclog with wire | Easy | Low | 85% |
| Drain Tube Disconnected | $0-$20 reattach | Easy | Low | 35% |
| Cracked Evaporator Case | $200-$800 case + 4-8 hrs | Hard | Medium | 20% |
| Heater Core Leak | $400-$900 heater core | Hard | Medium | 15% |
| Windshield Seal Leak | $100-$400 seal repair | Hard | Low | 10% |
| Sunroof Drain Clogged | $0-$30 unclog | Easy | Low | 8% |
| Cowl Drain Plug Clogged | $0-$10 clean cowl | Easy | Low | 5% |
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AC condensate drain is clogged. The evaporator drips about a cup of water per hour when AC runs. That water needs a drain tube to the ground. When the tube clogs, water overflows into the cabin.
Find the drain under the car (usually right of the engine, behind the firewall). Push a piece of weed-eater line or compressed air up through it. Black gunk comes out and water starts flowing freely.
Replace the cabin air filter on schedule and avoid parking under heavy leaf-shedding trees. Some people apply a small amount of bleach yearly to the drain to kill biofilm.
Coolant is sweet smelling, slightly sticky, and usually colored (green, orange, or pink). Pure AC condensate is odorless, colorless, and not sticky.
If left for weeks, yes. Mildew grows under the carpet padding. Pull the carpet, dry it, and reinstall before mold sets in.
Yes, that stops the condensate. But you lose AC. Better to unclog the drain, which usually takes 5 minutes.
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