Rinnai Tankless Water Heater Error Code 91: How to Fix It
Medium 30-45 minutes Medium Severity
Verified June 2026
- Error Code
- 91
- Brand
- Rinnai
- Product Type
- water_heater
- Severity
- Medium
- DIY Difficulty
- Medium
- Estimated Fix Time
- 30-45 minutes
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Tools You'll Need
- Phillips screwdriver
- Multimeter (optional)
How to Fix Error Code 91
-
Perform a Power Reset
Ensure the unit is powered off before touching any internal components. -
Check for Loose Wiring
Always disconnect power before opening the unit. High voltage components may be exposed. -
Inspect the Control Board
Do not touch the board while powered. Static electricity can damage sensitive electronics. -
Check for Water Leaks
Water and electricity are dangerous. Ensure the unit is dry before restoring power.
Parts You May Need
Main control board
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Main control board
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Main control board
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Main control board
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When to Call a Professional
If resetting and checking wiring does not clear the error, or if you see physical damage to the control board, contact a qualified Rinnai technician. The control board is a complex component that requires proper diagnosis and replacement by a professional.Frequently Asked Questions
What does Rinnai error code 91 mean?
Error code 91 indicates an internal data fault. It means the water heater's main control board has encountered a data processing error, often due to a temporary glitch or a hardware issue.
How do I reset my Rinnai tankless water heater?
To reset, turn off the power using the switch or unplug the unit. Wait 30 seconds, then turn the power back on. This clears many temporary error codes.
Can I fix error code 91 myself?
Yes, for simple causes like a power surge or loose wire. Try the power reset first, then inspect wiring. If the error persists, professional help is recommended.
Is error code 91 serious?
It's not immediately dangerous, but it can prevent your water heater from operating. If it occurs frequently, it may indicate a failing control board that needs replacement.
How much does it cost to fix Rinnai error code 91?
If it's a simple fix like a loose wire, the cost is minimal. If the control board needs replacement, expect $300-$600 for parts and labor, depending on your area.