Noritz error code 35 indicates an exhaust thermistor abnormality that typically requires checking wiring connections or replacing a faulty sensor.
Table of Contents
- What does error code 35 mean on a Noritz water heater?
- What usually causes error code 35 to appear?
- What are the quick fixes to try first?
- What are the next steps if the error persists?
- Which fix matches your specific symptom?
- When should you stop and call a technician?
- What should you have ready before you call?
- How can you keep this error from returning?
What does error code 35 mean on a Noritz water heater?
Noritz error code 35 indicates an exhaust thermistor abnormality where the control board fails to receive proper temperature signals from the exhaust sensor.
This error typically affects specific Noritz Pro Series and premix model lines. When the exhaust temperature sensor malfunctions or loses communication with the main circuit board, the unit safely shuts down to prevent overheating or combustion issues. Confirm your exact model manual to verify if this error applies to your specific unit.
- The exhaust thermistor monitors flue gas temperatures to ensure safe venting operation during combustion cycles.
- A loose wiring harness connection between the sensor and the circuit board can trigger this error code.
- Internal corrosion or moisture accumulation on sensor terminals may disrupt electrical signals and cause false faults.
- A damaged or frayed sensor wire can completely interrupt communication with the main control board.
- A completely failed thermistor component will require a physical replacement to clear the error code permanently.
What usually causes error code 35 to appear?
Error code 35 is most commonly caused by loose wiring harnesses, disconnected exhaust thermistor plugs, or a failing temperature sensor component.
Vibration from normal operation or thermal expansion can cause internal electrical connectors to loosen over time. Additionally, exposure to condensation inside the venting compartment can corrode terminal pins, preventing accurate temperature readings from reaching the control board. Identifying the root cause requires inspecting the internal wiring safely.
- Loose electrical plug connections directly behind the front panel on the main circuit board.
- Corroded or moisture-damaged pins on the exhaust thermistor wiring harness connector block.
- Physical damage or cuts along the wiring route leading from the exhaust sensor.
- Internal component failure inside the exhaust thermistor assembly due to continuous heat exposure.
- Temporary communication glitches that require a complete electrical power reset of the system.
What are the quick fixes to try first?
Perform a power reset and inspect the external wiring connections before attempting any internal component disassembly or sensor replacement.
Always turn off the circuit breaker supplying power to the water heater before touching any internal wires or components to prevent electrical shock. Simple power cycling can clear temporary digital glitches, while securing loose connectors often resolves communication errors instantly. Follow these safe preliminary steps carefully.
- Turn off the dedicated electrical circuit breaker supplying power to your water heater unit.
- Remove the front metal cover panel carefully using a standard screwdriver to expose internal wiring.
- Locate the exhaust thermistor wire harness running from the flue area to the control board.
- Unplug the wire connector, inspect for moisture or corrosion, and firmly push it back together.
- Turn the circuit breaker back on and test the water heater operation to observe results.
What are the next steps if the error persists?
Test the continuity of the thermistor wiring and check sensor resistance values with a multimeter to identify faulty components.
If basic connection checks fail to clear the error, deeper diagnostic testing is required. You will need a digital multimeter to measure electrical resistance across the sensor terminals and verify if the wiring harness is intact from end to end. Always ensure power remains disconnected during electrical testing.
- Disconnect the electrical circuit breaker completely before performing any resistance tests on internal parts.
- Disconnect the exhaust thermistor wiring harness entirely from the main circuit board connection point.
- Set your digital multimeter to measure ohms and test across the sensor connector pins.
- Compare your multimeter reading against the manufacturer resistance specifications listed in your service manual.
- Replace the entire exhaust thermistor assembly if the measured resistance falls outside normal operating ranges.
Which fix matches your specific symptom?
Use this reference table to match your exact observation with the corresponding likely cause and recommended repair action.
Different manifestation patterns can pinpoint whether the issue stems from a simple loose connection, moisture intrusion, or complete hardware failure. Reviewing these common scenarios helps determine whether you can fix the problem yourself or if professional support is required.
| Symptom | Likely cause / fix |
|---|---|
| Error appears immediately after power restoration | Temporary glitch; perform a hard power reset. |
| Error returns after jiggling internal wires | Loose wiring harness; secure or re-plug connector. |
| Error persists despite tight connections | Faulty thermistor; replace exhaust temperature sensor. |
| Visible corrosion on terminal pins | Moisture exposure; clean pins or replace harness. |
| Intermittent error during heavy usage | Failing sensor; test resistance with multimeter. |
| Error accompanied by burning smell | Wiring short; turn off power and call professional. |
When should you stop and call a technician?
Stop troubleshooting immediately and contact a licensed professional if you encounter gas leaks, electrical shorts, or unresolvable wiring damage.
Water heaters involve high-voltage electricity and potentially flammable gas connections. If basic wiring reseating does not resolve error code 35, or if you notice damaged wiring insulation or gas odors, professional servicing is essential for safety. Do not attempt advanced electrical repairs beyond your skill level.
- Any detection of gas odors or suspected gas valve leaks around the appliance.
- Frayed, burned, or severely damaged main circuit board wiring harnesses requiring professional replacement.
- Lack of confidence or experience when performing internal electrical resistance testing with a multimeter.
- Persistent error codes that return immediately despite confirmed secure connections and working sensors.
- Warranty protection requirements that mandate certified technician labor for component replacements.
What should you have ready before you call?
Gather your exact water heater model number, serial number, warranty documentation, and a record of troubleshooting steps attempted.
Having accurate appliance information ready accelerates support response times and ensures technicians bring the correct replacement parts. You can typically find the model and serial number rating plate affixed to the exterior side panel of your Noritz water heater unit.
- The exact model number printed on the silver specification rating plate sticker.
- The unique serial number associated with your specific water heater unit.
- Original purchase date and warranty registration details if the unit is relatively new.
- A clear summary of the troubleshooting steps you have already performed.
- Contact information and physical address where the water heater is installed.
How can you keep this error from returning?
Prevent future occurrences by scheduling annual maintenance, protecting outdoor units from moisture, and ensuring proper venting clearance.
Routine professional maintenance helps catch minor wiring wear and moisture accumulation before they trigger fault codes. Keeping your exhaust venting clean and free from blockages ensures optimal airflow and stable operating temperatures year-round during heavy daily usage.
- Schedule annual professional inspections to check internal wiring security and sensor performance.
- Ensure the venting system remains clear of bird nests, leaves, and physical obstructions.
- Protect outdoor-installed water heaters from severe weather and excessive direct moisture exposure.
- Inspect condensate drain lines periodically to prevent water backup near internal components.
- Keep the surrounding mechanical area clean and free from dust and corrosive chemical vapors.
Frequently Asked Questions
Can I still use hot water while error code 35 is displayed?
When error code 35 appears, the water heater safety mechanisms typically lock out operation to prevent potential hazards. Consequently, the unit will not ignite or heat water until the underlying exhaust thermistor issue is fully resolved. It is not recommended to force operation while safety faults are active.
What should I do if the error code comes back immediately after a reset?
If a power reset fails to clear error code 35, the issue is hardware-related rather than a temporary glitch. You should turn off the power breaker, open the front panel, and thoroughly inspect the exhaust thermistor wire harness. Loose or damaged wiring is the most probable culprit requiring attention.
Does this error code mean my entire water heater needs to be replaced?
No, error code 35 does not indicate a failing heat exchanger or total appliance failure. It specifically points to a localized communication or sensor issue within the exhaust monitoring circuit. Replacing the specific thermistor or securing the wiring harness will usually resolve the problem completely.
Are parts for Noritz water heaters covered under warranty?
Noritz typically offers a limited warranty covering parts and heat exchangers for specific durations depending on residential or commercial installation types. You should check your original purchase documentation or contact customer support with your serial number to verify active warranty coverage before buying replacement parts.
Is it safe to inspect the internal wiring of my gas water heater myself?
Yes, inspecting low-voltage sensor wiring is generally safe as long as you turn off the main electrical circuit breaker first. However, you must never touch gas valves, gas lines, or sealed combustion chambers yourself. If you feel uncomfortable working around electricity, always hire a licensed professional.
Bottom Line
Start by turning off the electrical breaker and safely checking the exhaust thermistor wiring harness connection behind the front panel. If securing the plug does not clear error code 35, test the sensor resistance or contact a professional technician for assistance.
Sources
- Noritz Support Knowledge Base
- Noritz Installation Manual
- Manufacturer Technical Service Guide