On the covered Rheem tankless gas water heater, 51 means error: a gas control valve malfunction. Rheem's pocket manual says the unit detected flame when demand for hot water ended and the unit turned off. First, turn off the gas at the shut-off valve.
Differential
Likely causes of “Rheem tankless gas water heater code 51”, and how to tell them apart
- 1
A residual flame from a blocked exhaust path
What separates it Code 51 together with a heater that is running hot or smelling of exhaust
How to confirm A restricted vent can keep combustion gases lingering and confuse a flame sensor
Before you touch a setting
Record this first — changing settings destroys the evidence
- Whether flame remains after the tap closes
- Whether you smell gas
Try in this order
0 of 9 completedWhat to do first
ReversibleTurn off the gas at the shut-off valve.
Correct only the documented fault domain
ReversibleRheem's pocket manual lists gas control valve diagnostics for a technician. Flame present after the burner should be out is a safety condition. Rheem's steps when a code shows: turn on a hot water faucet and recheck the remote control display. If the code stays, turn off the faucet and the water heater, unplug the water heater for about 30 seconds and plug it back in, follow the chart for your code, restart the water heater and recheck. If the code is still shown, turn off the faucet and the water heater, note the code and call for service. If a code is not in the chart, turn the water heater off immediately and call for service.
Turn off the gas
ReversibleAt the shut-off valve.
Turn off the water heater power
ReversibleThen wait.
Do not use the unit
ReversibleUntil serviced.
If you smell gas, leave
ReversibleAnd call the gas supplier.
Switch off the heater and note the time
ReversibleTurn the power and the gas off, and write down when code 51 appeared and what the heater was doing. A technician uses the time, the flow rate and the code history to decide whether the valve or the control is at fault.
Plan for hot water without the heater
ReversibleA tankless heater cannot be run safely with a gas control valve fault, so arrange another hot water source until the repair. A visit usually takes under an hour once the technician has the part.
Turn off the gas valve first
ReversibleRheem's pocket manual starts by closing the gas shut-off valve. Do that before you look at anything else, then leave diagnosis to a qualified technician.
Model-specific
What is true of this model and not of its siblings
- A safety lockout
- The unit expects the flame to go out when hot water stops. If it senses flame afterwards, it assumes the gas valve is leaking gas and locks out.
- Rheem's guidance
- Rheem's pocket manual lists gas control valve diagnostics for a technician and treats flame present after the burner should be out as a safety condition.
- What to do
- If you smell gas, leave the building and call your gas supplier; do not use switches or phones inside. Otherwise turn off the gas at the shut-off valve and call a licensed technician.
- The same condition as code 80
- Rheem's manuals list code 80 for the same condition on other models: a gas control valve malfunction in which flame is detected when demand ended and the unit turned off. If your unit shows 80, this page applies.
- Why it is a lockout
- A flame that persists after the valve should have closed means gas is flowing when it should not. The control locks out and refuses to restart until a technician has checked the valve.
- What to do now
- Turn off the gas at the shut-off valve, leave the unit off and call a licensed technician. If you smell gas, leave the building and call your gas supplier.
- Do not retry
- Do not turn the gas back on to see whether the code returns, and do not use the heater in the meantime. The lockout is protecting you from gas that may be leaking past a valve. Leave it off until a technician has tested the valve and the flame sensing.
- Who to call
- Call a licensed gas technician, not a general handyman. Gas valve work requires a leak test and combustion analysis after the repair, and many areas require a permit or inspection.
- Why flame after shutoff matters
- A burner that stays lit after the call for hot water has ended is using gas without a purpose and heating the exchanger with no flow through it. The control watches for exactly that and shows this code. Turn the heater off at the gas valve and call a technician at once. Do not wait to see whether it clears, because a valve that does not close fully can also leak gas.
Stop boundary
When this stops being a self-service repair
- !Turn off the gas at the shut-off valve
- !Flame after the burner should be out is a safety condition
- !Call a licensed technician
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 51 mean on a Rheem tankless gas water heater?+
Error: a gas control valve malfunction. Rheem's pocket manual says the unit detected flame when demand for hot water ended and the unit turned off.
Can I reset it?+
Turn off the gas first.
Who repairs it?+
A licensed technician.
How is code 51 different from code 80?+
Both involve the gas control valve, but each names a different circuit or condition, so give the technician the exact code. The first checks are the same: turn off the gas and call for service.
Can I reset the heater to get hot water?+
One reset is reasonable, but repeated resets of a gas valve fault are not safe, so stop after one.
Does hard water cause gas valve codes?+
Not directly, but scale reduces the heater's efficiency, so a yearly descale is worthwhile.
Is the valve repair covered by warranty?+
Many Rheem tankless heaters have a limited parts warranty, so have the serial number ready when you call.
Does code 51 appear after a long run?+
It appears when the heater sees flame after demand for hot water has stopped. That can follow a long run, since the valve has done the most work, but the check is the same either way.
What should I tell the technician?+
Say whether the code appeared at the end of a shower or after a brief draw, whether you noticed a smell, and whether the heater was recently serviced.
Is code 51 the same fault as code 80?+
They name the same condition in Rheem's list: the unit saw flame when demand ended. Which number shows depends on the model and control.
Can I try the heater again after turning it off?+
No. Treat a heater that may not shut its gas off as unsafe to run until a technician has tested the valve and flame sensor.