If you’ve suddenly lost your hot water and see “34” flashing on your controller, don’t worry! This is a common issue that is often caused by a simple sensor malfunction or a loose wire. It doesn’t mean your whole unit is broken; we just need to help the “brain” of the heater see the water temperature correctly again. Let’s get your hot showers back!
| Metric | Details |
|---|---|
| Difficulty | Intermediate (Requires use of a Multimeter) |
| Estimated Time | 45 – 90 Minutes |
| Tools Needed | Phillips head screwdriver, Digital Multimeter, Needle-nose pliers, Small towel |
| Estimated Cost | $25 – $65 (If a new thermistor is required) |
Symptoms
When your Rinnai unit encounters Error 34, it will exhibit several distinct physical signs. Recognizing these will help confirm you are dealing with a thermistor issue:
🛠️ Safety Precaution: High Voltage
Don’t guess the wiring. Get the official PDF to see the exact schematics.
- The Digital Display: Your Rinnai remote controller or the display on the front of the unit will be flashing a bright “34” and may beep to alert you of the lockout.
- The “Cold Sandwich” or No Heat: You might be getting nothing but cold water, or perhaps the water starts to get warm for a few seconds before suddenly turning ice cold as the unit fails its internal safety check.
- Audible Clicking: You may hear the unit attempt to ignite (a series of clicks), but it will quickly shut down and go silent because the computer doesn’t know the incoming water temperature.
- Fan Operation: Sometimes the internal combustion fan will spin at high speed for a moment as a purge cycle, but the burners will never actually stay lit.
Technical Explanation of the Fault
To fix the problem, it helps to understand what is happening inside the “heart” of your water heater. The inlet thermistor is a negative temperature coefficient (NTC) resistor. In plain English: it’s a tiny probe that changes its electrical resistance based on how cold or warm the water is. The Rinnai control board sends a small voltage to this sensor; if the resistance it meets is within a “normal” range, the heater works. If it’s not, you get Error 34.
Why do these fail? Here are the primary culprits:
- Natural Wear and Tear: These sensors live in a harsh environment. They are constantly subjected to thermal expansion and contraction as water temperatures change. Over several years, the internal ceramic element can develop microscopic cracks, causing the sensor to provide “garbage” data to the control board.
- Mineral and Scale Accumulation: If you live in an area with hard water, calcium and magnesium can coat the tip of the thermistor. This creates an insulating “sleeve” around the sensor, making it unable to read the water temperature accurately or quickly enough, leading to a timing error in the board.
- Voltage Spikes or Electrical Noise: A nearby lightning strike or a surge in your home’s electrical grid can occasionally “fry” the delicate circuitry of the thermistor or the specific port on the Printed Circuit Board (PCB) that reads it.
- Corrosion at the Connection: Because the water heater involves both water and electricity, humidity can sometimes cause “green” oxidation (corrosion) on the wire harness pins. This creates high resistance that the computer interprets as a failed sensor.
How to Fix Rinnai Error 34 (Step-by-Step)
Don’t worry, you can do this! We are going to move slowly and safely. If at any point you feel uncomfortable, there is no shame in calling a pro, but let’s try to save you that service fee first!
- Safety First (Power and Gas):
Before you even touch a screwdriver, pull the electrical plug for the water heater from the outlet. Then, find the yellow handle on the gas line leading to the unit and turn it 90 degrees so it is perpendicular to the pipe (the “Off” position). Safety is our top priority! - Remove the Front Access Panel:
Locate the four screws (usually two at the top and two at the bottom) holding the front metal cover in place. Using your Phillips head screwdriver, remove them and set them in a safe place (like a magnetic tray or a cup). Gently pull the cover toward you and lift it off. Place the cover somewhere it won’t get scratched. - Locate the Inlet Thermistor:
Look at the bottom of the unit where the cold water pipe enters. Follow that pipe upward into the unit. You will see a small plastic plug with two wires (often white or black) inserted into a brass or plastic housing on that pipe. This is your target. - The Multimeter “Health Check”:
Unplug the wire harness from the thermistor. Set your multimeter to the “Ohms” (Ω) setting. Place your probes on the two metal pins of the thermistor itself (not the wires leading to the computer).
Tip: At room temperature (approx. 68°F), you should see a reading of about 10k to 15k Ohms. If the meter reads “0” (Short circuit) or “OL” (Open circuit/dead), the sensor is definitely bad and needs replacement. - Inspect and Clean Connections:
If the resistance looks okay, look closely at the plug and pins. Do you see any green crust or white powder? If so, use a bit of electronic contact cleaner or gently scrape the pins with a needle to ensure a shiny metal-to-metal connection. Sometimes just unplugging and replugging it firmly fixes the “ghost” Error 34. - Replacing the Thermistor (If Failed):
If the test in Step 4 failed, you need a new part.
WARNING: You must drain the water heater before removing the thermistor, or you will have a flood inside your unit!
Close the cold water inlet valve. Open a hot water tap in the house to bleed pressure. Place a towel under the unit. Use your needle-nose pliers to remove the metal “C-clip” holding the sensor in place. Pull the old sensor out, slide the new one in, and re-insert the clip. - Reassemble and Test:
Plug the wire harness back into the sensor. Turn your gas back on. Plug the unit back into the wall. Turn on a hot water tap and see if the unit fires up. If it works, celebrate! Put the front cover back on and secure those four screws.
How to Prevent Error 34
Now that you’ve got your hot water back, let’s make sure this doesn’t happen again anytime soon. Maintenance is the secret to a long-lived Rinnai!
- Annual Descaling (The “Flush”): Hard water is the enemy of tankless heaters. Every 12 months, you should flush your unit with food-grade white vinegar using a small pump kit. This removes the scale buildup that can insulate and damage your thermistors.
- Install a Surge Protector: Since the Error 34 can be caused by electrical “noise” or spikes, plugging your water heater into a high-quality surge protector (specifically one rated for appliances) can protect the delicate sensors and the expensive main PCB from power fluctuations.
- Keep it Clean and Dry: Periodically check inside the cabinet for any signs of moisture or insect nests. Spiders love to build webs near sensors, and moisture can lead to the corrosion we discussed earlier. A quick vacuuming of the bottom of the cabinet (while powered off!) goes a long way.
FAQ
Frequently Asked Questions
Q: Can I bypass the thermistor just to get one hot shower?
A: Absolutely not. The thermistor is a vital safety component. If the unit doesn’t know how hot the incoming water is, it could fire the burners too high and potentially scald you or damage the heat exchanger. The system is designed to lock you out for your own protection.
Q: How much does a professional charge to fix Error 34?
A: Typically, a plumber or HVAC tech will charge a service call fee ($100–$200) plus the cost of the part (marked up to $80–$100). By doing it yourself, you are likely saving between $150 and $300.
Q: I replaced the thermistor, but I still see Error 34. What now?
A: If a brand-new sensor doesn’t fix it, there may be a break in the wire harness leading back to the control board, or the control board (PCB) itself has a failed circuit. Check the entire length of the wires for any chew marks from rodents or accidental pinches.