Rheem Tankless Gas Code 34
Ambient thermistor malfunction. The sensor reading the air temperature around or inside the unit is not reporting correctly. It is used for freeze protection and for compensating combustion against conditions, rather than for making hot water — so the heater usually keeps working.
Part replacementThe verdict
Replace a part
The least urgent of Rheem's thermistor codes for hot water, and the most urgent for winter: this is the sensor freeze protection leans on. Get it fixed before cold weather rather than after.
Typical cost: $150–$450
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Tankless Gas — not averaged across the brand.
Source: Rheem Tankless Gas Troubleshooting Pocket Manual (SVC820) (p. 64), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Book the repair before freezing weather — freeze damage is excluded from essentially every manufacturer's warranty.
- If the unit is outdoors or in an unheated space, treat this as more urgent.
Frequently asked questions
The heater still makes hot water. Can it wait?
Through summer, probably. Into winter, no — freeze protection is what stops a burst heat exchanger, and it needs this sensor to know when to start.
Don't confuse this with
4 other Rheem Tankless Gas codes point at a thermistor too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: