Improper thermistor connection
The A035 error code on Rheem boilers indicates improper thermistor connection.
Rheem A035 means: improper thermistor connection. The usual causes are gas valve coil failure (electrical fault), gas valve stuck closed or partially open or gas supply interrupted (prepayment meter, emergency control valve). A Gas Safe registered engineer should diagnose the root cause.
What Does A035 Mean
The A035 code is shown by the boiler’s control board when it detects improper thermistor connection. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows A035 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 2 technical sources.
| Cause |
|---|
| Gas valve coil failure (electrical fault) |
| gas valve stuck closed or partially open |
| gas supply interrupted (prepayment meter, emergency control valve) |
| incorrect gas pressure (too high or too low) |
| gas valve wiring fault |
| PCB not sending open signal to the gas valve |
Safety Warning
A035 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Warning: this fault code may relate to gas supply, flue or combustion issues. Do not attempt repair — contact a Gas Safe registered engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Consult service manual
Refer to the Rheem service documentation for the specific diagnosis procedure for fault code A035.
Check error log
Access the boiler error log to check for related fault codes that may help identify the root cause.
Contact manufacturer support
If the fault persists after basic checks, contact Rheem technical support for specialist guidance.
How to Fix It
Check the gas valve connector and wiring for continuity, then test the valve coil resistance against the manual; replace the valve if it is out of range.
Check that the gas service valve is open and that other gas appliances in the property work. If they do not, the problem is the gas supply, not the boiler.
Have the inlet gas pressure measured at the boiler with a manometer and compared against the figure in the manufacturer’s data table.
Unplug and re-seat the connector of the gas valve wiring, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Rheem service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Rheem models that can display fault code A035:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Rheem models | — | Possible |
Frequently Asked Questions
A035 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.