59 Error on Rheem Furnace? Comprehensive Fix Guide

Rheem Error Code 59 is a specific diagnostic signal indicating that water has been sensed in the collector box. In high-efficiency condensing furnaces, the combustion process produces acidic moisture that must be drained away; Error 59 triggers when the safety sensor detects that this water is backing up, threatening to flood the inducer motor or the burner assembly.

If you are encountering this code, you are likely experiencing a complete lack of heat, perhaps accompanied by a rhythmic “gurgling” sound emanating from the furnace cabinet. While a sudden loss of heating in cold weather is stressful, don’t worry—this is a protective safety shutdown. It is often caused by a simple blockage in the drainage system rather than a catastrophic component failure, and it is highly fixable with the right approach.

Symptoms of Rheem Error 59

• Digital Display Flash: The 7-segment LED display on the furnace control board will distinctly flash the numbers “5” then “9” repeatedly.
• Ignition Failure: The furnace may start its initial cycle (you will hear the inducer motor spin up), but it will never click over to ignite the burners.
• Audible Gurgling: You may hear the sound of water splashing or air bubbling through a liquid-filled chamber inside the furnace cabinet.
• Blower Motor Continuous Operation: In many software versions, the furnace will run the indoor blower motor constantly to help dry out the cabinet, even if no heat is being produced.
• Visible Moisture: You might notice small pools of water at the base of the furnace or dampness around the PVC drain lines.

Why is my Rheem showing Error 59?

Understanding the “why” requires a look at the thermodynamics of a condensing furnace. Unlike older models, a high-efficiency Rheem captures heat from exhaust gases until they turn into liquid condensate. This liquid must exit the system via a collector box and a series of tubes.

• Clogged Condensate Trap: This is the most common culprit. Over years of operation, “white slime” (biological growth or mineral deposits from the acidic water) builds up inside the internal plastic trap. Once the trap is fully restricted, water backs up into the collector box where the sensor is located.
• Blocked External Drain Lines: If the PVC pipe exiting your furnace is clogged with debris, or if it is a cold winter and the pipe has frozen outside, the water has nowhere to go. This mechanical backup forces the water level to rise until it touches the probe.
• Sensor Corrosion or Fouling: The sensor itself is a metal probe. Because the condensate is acidic, the probe can develop a layer of oxidation or “scale.” This scale can sometimes bridge the gap to the ground or hold moisture against the probe, creating a “false positive” reading even if the water has drained.
• Improper Venting Pitch: If the exhaust vent (the PVC pipe going outside) is not pitched correctly back toward the furnace, or if it has a “sag” in the line, water will collect in the pipe. Eventually, a large “slug” of water can rush back into the collector box all at once, overwhelming the drain and tripping Error 59.

Troubleshooting & Replacement Instructions

1. Safety First – Power and Gas: Before opening the cabinet, locate the furnace power switch (looks like a light switch) and flip it to ‘OFF’. For extra safety, shut off the gas valve leading to the unit. WARNING: Working on a furnace with the power on can result in electrical shock or damage to the integrated furnace control (IFC) board.
2. Access the Collector Box: Use your 5/16″ nut driver to remove the screws holding the upper and lower cabinet doors. Look for the large plastic “collector box” located behind the inducer motor. You will see a small wire (usually yellow or orange) leading to a single-pole sensor pushed into this box.
3. Inspect and Clean the Water Sensor: Remove the screw securing the water sensor to the collector box. Carefully pull the sensor out. If the tip of the metal probe is covered in black soot or white crust, use a piece of steel wool or fine-grit sandpaper to clean it until the metal is shiny. This ensures a proper electrical reading. Re-insert it carefully to ensure the gasket creates a tight seal.
4. Clear the Condensate Trap: Locate the plastic trap (usually a black or translucent box inside or just outside the furnace). Disconnect the rubber hoses attached to it. CAUTION: These hoses will likely be full of water; have a bucket ready. Take the trap to a sink and flush it with warm water and a small amount of bleach or vinegar to break down any slime. If you cannot clear it by hand, use a wet/dry vacuum to suck out the debris from the drain lines.
5. Check the Drain Lines for Obstructions: Attach your wet/dry vacuum to the end of the PVC drain line outside your house. Seal the gap with a rag and run the vacuum for 30 seconds. This will pull out any “slugs” of dirt, bugs, or ice that are preventing the furnace from breathing and draining.
6. Test the Sensor Continuity: If the error persists after cleaning, use a multimeter set to Ohms (Resistance). Check the continuity between the sensor terminal and the furnace ground. If the sensor shows a closed circuit to ground when dry, the internal insulation of the sensor has failed, and the part must be replaced.
7. System Restart: Reattach all hoses, ensuring they are tight. Restore the gas and power. The furnace will go through a self-test. If the water has been successfully drained and the sensor cleaned, Error 59 should clear, and the ignition sequence will begin.

How to Prevent Error 59

Error 59 is almost always a maintenance-related issue. By taking proactive steps, you can ensure your furnace doesn’t shut down during the coldest night of the year.

• Annual Trap Flushing: Once a year, before the heating season starts, remove the condensate trap and flush it with a mixture of 1 part bleach to 10 parts water. This prevents the “slime” buildup that leads to most drainage codes.
• Install a Condensate Neutralizer: Since furnace water is acidic, it can eat away at your plumbing and create sediment. A neutralizer kit buffers the PH of the water, making it less likely to cause mineral buildup in your furnace’s internal drainage components.
• Verify Pipe Pitch: Ensure that all horizontal runs of PVC vent pipe have a minimum slope of 1/4 inch per foot back toward the furnace. This allows gravity to assist the drainage process and prevents water from “pooling” in the vents.

Frequently Asked Questions

Can I bypass the water sensor to get heat temporarily?
It is strongly discouraged. The sensor is there to protect the inducer motor and the heat exchanger. If you bypass it and water continues to back up, you could flood the combustion chamber, leading to a much more expensive repair (over $1,000) or a fire hazard due to electrical shorts.

Why does Error 59 happen more often in extremely cold weather?
When temperatures drop below freezing, the condensate water inside the PVC drain line that exits your home can freeze. This creates an ice plug. Because the water cannot exit the house, it backs up into the furnace collector box, tripping the sensor.

Is Error 59 the same as a Pressure Switch error?
No, but they are related. A pressure switch (Error 31) usually detects air flow issues. Error 59 specifically looks for the physical presence of liquid water. However, a clogged drain can sometimes cause enough water backup to also trip a pressure switch because the air can no longer pass through the collector box.