Troubleshooting Rinnai Error Code 58: What It Means & How to Fix

A Rinnai Error 58 indicates a **Burner Sensor Error**. Specifically, it means the unit’s control board has detected an abnormal signal from the burner sensor (thermocouple or flame rod) during operation. This is a safety lockout; the system stops the gas flow because it can’t confirm the burner is operating safely.

If you’re staring at a flashing “58” on your controller, you’re likely dealing with a sudden blast of ice-cold water mid-shower or a unit that clicks repeatedly but never actually fires up. Look, I’ve been under the hood of these Rinnai units for over twenty years, and while a sensor error sounds intimidating, it’s usually just a case of the machine needing a bit of “mechanic’s TLC.” It’s a common hurdle for aging units, but we can usually get you back into a hot shower without replacing the whole system.

Troubleshooting & Replacement Instructions

Alright, let’s get our hands dirty. Follow these steps in order. Most of the time, Step 4 will solve your problem without costing you a dime in parts.

1. Safety First (Non-Negotiable): Before you touch anything, turn off the electrical power to the unit at the disconnect or unplug it. Then, reach down and turn the gas ball valve to the “Off” position (perpendicular to the pipe). You’re working near gas and high voltage; don’t skip this.
2. Remove the Front Cover: Use your Phillips screwdriver to remove the four screws holding the front panel on. Be careful as you pull the panel away; sometimes the display controller wire is attached to the front. Gently unplug that wire if necessary and set the panel aside in a safe spot so it doesn’t get scratched.
3. Locate the Burner Sensor: Look at the large silver combustion chamber in the center. You’ll see several wires leading to the top or side of the burner tray. Look for a single wire (often yellow or orange) leading to a small metal rod inserted into the burner. This is your burner sensor/thermocouple.
4. The Cleaning Trick (The “Veteran Move”): Remove the screw holding the sensor in place and carefully pull it out. WARNING: Do not drop the tiny gasket or screw into the bottom of the unit. Inspect the metal rod. If it looks dull, white, or black, take your steel wool or fine sandpaper and lightly scrub the rod until the bare metal shines. Do not use heavy grit sandpaper; you don’t want to gouge it, just polish it. Reinstall it and try to fire the unit up.
5. Check Continuity and Voltage: If cleaning didn’t work, grab your multimeter. Set it to Ohms (Resistance). Check the wire leading from the sensor back to the PC board. You’re looking for a “closed loop” (near zero ohms). If you see “OL” (Open Loop), the wire or the sensor is physically broken inside. You’ll need to order a Rinnai-specific replacement sensor kit.
6. Inspecting the PC Board: Check the wiring harness where it plugs into the board. Look for any scorched marks or “fried” smelling components. If the sensor is clean and the wiring has continuity, but Error 58 persists, the logic gate on the board is likely dead, requiring a board replacement.

Why is my Rinnai showing Error 58?

When I see this code on a job site, my mind immediately goes to four specific culprits. These aren’t just random guesses; they are the result of how these machines handle high-heat combustion over years of service.

1. Carbon and Mineral Buildup (Sooting): This is the #1 cause. The burner sensor sits right in the path of the flame. Over time, microscopic impurities in the gas or dust in the combustion air create a thin layer of carbon or white silica on the sensor rod. This “crust” acts as an insulator. The sensor is trying to send a micro-amp signal to the PC board, but the buildup blocks it. The board thinks there’s no flame and kills the gas for safety.

2. Connector Corrosion or Loose Wiring: Tankless units vibrate when the fan is at high RPMs, and they go through extreme heat-cool cycles. This can cause the spade connectors on the burner sensor to loosen or develop oxidation. If the connection is weak, the voltage drop is enough to trigger Error 58. I’ve seen plenty of “broken” heaters that just needed a wire pushed back on tight.

3. Thermal Fatigue: The sensor itself is a piece of metal subjected to thousands of degrees. Eventually, the internal resistance of the thermocouple changes, or the ceramic insulator cracks. Once that ceramic is compromised, the signal can “leak” to the metal chassis (grounding out) before it reaches the control board.

4. PC Board Interpretation Error: Though rare, the “brain” of the unit can fail. If the board’s internal circuitry that processes the flame signal burns out (often due to a power surge), it won’t matter how clean the sensor is; the board will remain blind to the flame.

Symptoms of Rinnai Error 58

In my experience, Error 58 doesn’t usually happen “quietly.” You’ll notice the unit struggling before it finally gives up the ghost and throws the code. Here is what to look for:

• The “Start-Stop” Cycle: You might hear the fan kick on and the igniter clicking (a rapid tick-tick-tick sound), but the unit shuts down after only a few seconds of flame.
• Fluctuating Water Temps: Before the hard lockout, you might experience “cold water sandwiches”—where the water gets hot, goes cold for a minute, and then gets hot again as the sensor fails intermittently.
• The Flashing Display: The digital controller will bypass the temperature setting and flash “58” repeatedly. Even if you power cycle the unit, the code will likely return as soon as the burner attempts to ignite.
• Audible Relays: You might hear a distinct “clunk” of the gas solenoids closing immediately after the unit fails to sense the burner’s status.

How to Prevent Error 58

If you don’t want to be back inside this unit in six months, you need to change how you treat it. These machines are high-performance engines; they need maintenance.

• Annual Descaling and Cleaning: Most people know about flushing the heat exchanger with vinegar, but they forget the “air side.” Use a vacuum or compressed air to clean the fan intake and the burner area. Dust is the enemy of a clean flame. A dirty flame creates the soot that kills your sensor.
• Install a Gas Sediment Trap: If you don’t have a “drip leg” on your gas line, debris from the gas pipes can enter the burner, causing uneven heating and sensor fouling. It’s a simple plumbing addition that saves the burner assembly.
• Use a Dedicated Surge Protector: The PC boards in Rinnai units are sensitive. A small power spike can damage the sensor’s circuit. I always tell my clients to plug their tankless unit into a high-quality surge protector, not just a standard outlet.

Frequently Asked Questions

Q: Can I bypass the burner sensor just to get a hot shower tonight?
A: Absolutely not. Never, under any circumstances, should you bypass a safety sensor. The sensor is there to ensure gas isn’t pumping into your home without a flame to burn it. Bypassing this is a recipe for an explosion or carbon monoxide poisoning. If it’s broken, fix it right.

Q: My sensor is clean, but Error 58 keeps coming back. What now?
A: Check your venting. If your intake or exhaust pipes are partially blocked (bird nests are common), the “air-to-fuel” ratio gets wonky. This creates a “lazy” flame that doesn’t properly engulf the sensor rod, triggering the error even if the sensor is brand new.

Q: How much does a professional charge to fix Error 58?
A: Typically, a service call ranges from $150 to $300 plus parts. If it’s just a cleaning, you’re mostly paying for the technician’s time and expertise. If the sensor or board needs replacement, expect the bill to climb toward $400-$600. That’s why I recommend trying the “steel wool” cleaning method yourself first!