You turn on the hot tap, the Rheem clicks like it's trying to start, and the display flashes 11. No hot water. Code 11 is one of the most common faults on Rheem and its rebadged cousins, and the good news is that it points at a short list of specific things — not a mystery. It means ignition failure: the unit tried to light the burner and never confirmed a flame. Half the time the fix is free. The trick is checking the causes in the right order instead of guessing.
SHORT VERSION
Code 11 = ignition failure: the burner tried to light and no flame was sensed, so the unit shut the gas. Check in order: gas on and flowing (do other gas appliances work?), venting clear (intake/exhaust not blocked), flame rod clean, and gas pressure adequate. Reset after each. Most fixes here cost nothing.
What code 11 is telling you
Every gas tankless follows the same startup handshake. Flow sensor sees water moving, the board runs the fan, opens the gas valve, and fires the igniter. A flame rod — a thin metal probe in the burner — then has to confirm an actual flame within a second or two by sensing the tiny current a flame conducts. If it doesn't get that confirmation, the board shuts the gas off and posts a lockout code. On Rheem, that lockout is 11.
So code 11 always means one of two things: the burner never lit, or it lit but the rod didn't sense it. Everything you check is aimed at telling those two apart and finding why. Because it's a lockout, the unit will keep showing 11 and refuse to run until you reset it and it either succeeds or fails again — which is exactly what makes it diagnosable.
Work the causes in order
Run these top to bottom. After each one, reset the unit (controller off and on, or cycle power) and try hot water again, so you know exactly which step changed things.
Confirm gas is on and flowing
The most common cause is the simplest: no gas or not enough. Make sure the manual gas valve at the unit is fully open and the meter or propane supply is on. Then light another gas appliance — a stove burner, the furnace. If it's weak or won't light either, the problem is supply-wide, not the heater.
Check the venting
A blocked intake or exhaust starves or backs up the burner and stops ignition. Look for birds' nests, snow or ice at the terminations, crushed pipe, or a vent run that's simply too long or has too many elbows. On a fresh install, an over-length vent is a classic code 11.
Clean the flame rod and igniter
If gas and venting are good, the burner may be lighting but going undetected. Power down, open the unit, and inspect the flame sensing rod and igniter for soot, white corrosion or a cracked insulator. Light abrasive on the rod and a clean ground connection often restore sensing.
Verify gas pressure
A failing regulator, undersized gas line, or a nearly empty propane tank leaves too little pressure to light under demand. If other appliances also run weak, or the code returns only during heavy hot-water use, suspect pressure. Measuring it properly is a job for a pro with a manometer.
The two-appliance test that saves time
Before you open anything, do one thing: run another gas appliance. This single check splits the whole problem in half. If the stove lights strong and the furnace fires while the tankless throws 11, you know the house has gas and reasonable pressure — the fault is at the heater, so move to venting and the flame rod. If the stove is weak, slow to light, or the furnace also struggles, the problem is upstream: a supply, regulator or pressure issue affecting everything, and cleaning the tankless flame rod won't touch it. Two minutes here tells you which direction to spend the next hour.
Match the symptom to the cause
| What you're seeing | Most likely cause | Fix & cost |
|---|---|---|
| No click, no attempt to light | Gas off or supply issue | Reopen gas, $0 |
| Clicks, won't light, other gas weak | Low gas pressure | Service call |
| New install, never ran right | Vent too long / undersized gas | Correct install |
| Lights then locks to 11 | Dirty flame rod / bad ground | Clean, $0–80 |
| Intermittent under heavy use | Marginal pressure or flame rod | Diagnose both |
It cleared after a reset — am I done?
Maybe not. A code 11 that clears and then returns, especially during a long shower or when another appliance kicks on, is telling you the fault is intermittent. That pattern almost always means gas pressure dropping under load or a flame rod that senses when clean but fades as it heats. Watch when it comes back — the timing is the clue.
When it's the install, not the unit
If a tankless has thrown code 11 since the day it went in, stop chasing parts — look at the installation. Tankless burners need a lot of gas fast, and the two things installers get wrong are the gas line and the vent. A gas line sized for the old tank heater may be too small to feed a tankless at full fire, so it lights fine on a trickle of hot water and locks out on a full-demand call. Venting has strict length and elbow limits; a run that's too long, or one elbow too many, chokes ignition. Neither is a repair you make by swapping a sensor. If the unit is new and code 11 shows up under real demand, get the installer back to check gas-line sizing and vent length against Rheem's spec — that's where the fault lives.
What it costs to clear
Code 11 has one of the widest cost ranges of any fault, which is why order matters so much. Reopening a gas valve, clearing an iced vent terminal, or cleaning the flame rod costs nothing but time, and these solve a large share of cases. A flame sensor or igniter is $30 to $80 in parts if you're comfortable inside the unit. The expensive end is a gas control valve or control board, or an install correction like re-sizing the gas line — combined with a service call, a pro repair on a tankless commonly runs $200 to $400. Working the free checks first means you only pay for the big stuff if you've truly ruled out the small stuff.
Frequently asked
What does code 11 mean?
Ignition failure — the unit tried to light and no flame was sensed, so it shut the gas. Causes: no or low gas, blocked venting, a dirty flame rod, or an igniter/valve fault.
How do I fix it?
In order: confirm gas is on and other appliances light, clear the intake/exhaust vents, clean the flame rod and igniter, then check gas pressure. Reset after each step.
It lights then shows 11 — why?
The flame rod isn't sensing the flame it made. Usually a dirty or corroded sensor, a cracked insulator, or a poor ground. Clean the rod and confirm the ground.
Can low gas pressure cause it?
Yes. An undersized line, failing regulator or near-empty propane tank starves the burner. If other gas appliances also run weak, suspect supply-wide pressure.
How do I reset the unit?
Turn the controller off and on, or cycle power, to clear the lockout and let it re-attempt ignition. Reset after each check so you can see what changed.
What will a repair cost?
Often nothing — reopening gas, clearing a vent, cleaning the rod. A flame sensor/igniter is $30–80; a valve, board or install fix with a service call runs $200–400.
Different code, or an older tank instead?
Start from the full library of water heater fixes and find the one that matches your symptom.