Rheem Furnace Code 59: Water Sensed in the Collector Box
What the code means
In Rheem's fault code table for the R96V condensing furnace (installation instructions 92-24161-124-01, Table 18), code 59 is "Water Sensed," a critical fault. The furnace control (IFC) watches for current flowing from pins 1 and/or 2 of its P4 connector. Those pins go to water sensors in the collector box. When water is present, a sensor passes a small current to the furnace sheet metal, which tells the control water is backing up. The condition has to be present continuously for at least ten seconds before the control declares the fault. While it's active, no gas heating can run and the code is displayed; cooling and other modes should work normally. If water is sensed during a heat call, clears, and is sensed again within 5 minutes of the next heat attempt in the same call, the control goes into a one-hour soft lockout and the display alternates "59" and "10" (soft lockout). Lockout resets on its own after one hour. Code 59 is different from code 58, which means the water sensor circuit is open rather than detecting water.
Likely causes
- A blocked condensate drain line, or a drain trap that's clogged and can't let condensate water flow out (Rheem's first listed cause)
- Sensor wiring that's been damaged so bare wire touches the furnace sheet metal, which looks like water to the control
- A water sensor that's been pulled out of the collector box with its wires still attached, so its metal probe is touching the cabinet
- A drain trap or drain pipe that froze, in an attic, crawl space or other spot that drops below 32°F (Rheem requires freeze protection for traps outside the cabinet)
Safe DIY checks
Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.
- Turn the thermostat off and switch off furnace power before opening the panels.
- Find the condensate trap and drain hose on the side or front of the furnace. Look for standing water, sludge or a kinked or sagging hose.
- Clear the drain: disconnect the hose and flush it, and rinse out the trap if your model lets you remove it. If the drain runs to a condensate pump, make sure the pump isn't full or dead.
- If the furnace is in an attic, garage or crawl space, check whether the trap or drain line froze. Thaw it, and plan freeze protection with a pro.
- Look at the wires running to the water sensors on the collector box. If a sensor is hanging loose or a wire is nicked and touching metal, leave the repair to a technician.
- Restore power and set a heat call. If 59 and 10 alternate, the furnace is in a one-hour lockout that resets on its own once water is gone. If the code keeps coming back with a clear drain, call an HVAC technician.
🛠 When to call a pro
- Removing the blockage from the condensate drain, trap or collector box drain hoses and confirming water drains freely.
- Repairing or replacing damaged wiring between the furnace control and both water sensors.
- Returning a loose water sensor to its proper place in the collector box.
- Adding freeze protection to the drain trap and drain pipe where the furnace is in an unheated space.
- Checking the drain hoses and trap are routed and sloped correctly for the furnace's orientation.
Parts that commonly fix it
Always match the part number to your exact model.
Condensate drain hose
Replace a cracked, kinked or sludged-up drain hose. Match the diameter on your furnace.
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Good insurance for any gas furnace, especially after drain and venting work.
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Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.