33
Ruud
Heat Exchanger Thermistor Fault
Ruud tankless water heater displays error code 33; unit may not modulate correctly.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Failed heat exchanger thermistor, Damaged thermistor harness, Poor connection at control board
How to Fix / Troubleshooting
Safety first: Turn OFF power and gas. Allow the unit to cool.
- Step 1 – Locate heat exchanger thermistor: Remove the front cover and identify the thermistor mounted directly on the heat exchanger body.
- Step 2 – Inspect wiring: Look for damage or loose connectors. Reseat the plug at both ends.
- Step 3 – Replace thermistor: Remove the mounting screw or clip, pull out the sensor, and install a new OEM Ruud heat exchanger thermistor. Reconnect wiring.
Restore power and test. If code 33 persists, the control board may need professional evaluation.
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Repair Difficulty
Medium
3/5
Required Part
Heat Exchanger Thermistor
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