What Rheem Error Code 11 Means
Rheem error code 11 = ignition failure (no ignition). The control board opened the gas valve, fired the igniter, and waited — but the flame sensor rod never confirmed a stable flame. The unit tried up to three times, then locked out and displayed code 11.
On every RTG and RTGH series Rheem tankless water heater, the ignition sequence follows a fixed chain: the flow sensor detects water movement above 0.5 GPM, the combustion fan spins up to proving speed, an air-pressure switch confirms combustion airflow, the gas valve opens, the igniter electrode sparks, and the flame sensor rod must detect a stable flame within approximately three seconds. If that final confirmation never arrives — because the gas did not reach the burner, the spark was too weak, or the sensor rod was too corroded to conduct — the board closes the gas valve and logs error code 11.
This is a safety lockout, not permanent damage. The unit is waiting for you to identify and fix the broken link in that ignition chain, then perform a reset. In most cases the fix takes under 20 minutes and requires no parts.
Nearby Rheem codes for context
No ignition / ignition failure
This guide
Flame lost during operation
Related
Combustion air supply failure
Related
7 Most Common Causes of Rheem Code 11 — No Ignition
Work through these in the order listed. The first two causes resolve about 65% of all code 11 calls.
- gas_meterCheck first#1
Closed or low-pressure gas supply
The burner cannot ignite without adequate gas. A closed shutoff valve, low line pressure (under 3.5" WC for NG), or a main-line interruption will trigger code 11 every time.
- sensors_offMost common#2
Dirty or oxidized flame sensor rod
The rod detects the flame via electrical conductivity. Oxidation insulates the tip, making the control board 'think' ignition never happened — even when the burner briefly lit. Cleaning resolves ~45% of all code 11 calls.
- airCheck third#3
Blocked air intake or exhaust vent
Debris, ice, bird nests, or a crushed vent section starves the burner of combustion air. The unit may also abort ignition if the air-proving pressure switch cannot close.
- filter_altEasy fix#4
Clogged cold-water inlet filter screen
Scale and sediment on the mesh filter screen drop flow below the 0.5 GPM activation threshold. The unit won't fire at all — no ignition sequence is triggered without adequate flow.
- water_dropCheck fifth#5
Water flow rate below minimum threshold
Even with a clean filter, a partially closed valve or a low-flow aerator can hold flow at or below 0.5 GPM — right at the edge of Rheem's activation threshold.
- electric_boltTest it#6
Worn or cracked igniter electrode
A spark gap wider than 4 mm produces a weak spark. A cracked ceramic insulator grounds the spark before it reaches the gap. Either way: no reliable ignition.
- hardwareCall a pro#7
Faulty gas valve solenoid or control board
If all DIY checks pass and code 11 persists, the gas valve may not be opening under the PCB's command, or the PCB ignition driver circuit has failed. These require professional testing.
Pro tip
Note your model and serial number before starting — it's on the rating plate inside the front panel. You'll need it if ordering parts or calling Rheem support (1-800-432-8373).
Step-by-Step Diagnosis — How to Find the Real Cause
Total time: 20–30 minutes for a complete DIY diagnosis.
Tools needed: flat-head screwdriver, 0000 steel wool, flashlight. Optional: digital multimeter, feeler gauge for advanced testing.
Check the gas supply and line pressure
schedule2–3 minGo to the gas shutoff valve on the line feeding your water heater. The handle must be parallel to the pipe — not perpendicular. Light a burner on your stove or turn on your furnace to confirm the main supply is live. If you have a manometer, verify static pressure is ≥ 5" WC (natural gas) or ≥ 11" WC (LP).
Reset the unit
schedule1–2 minPress the Reset button on the front panel (hold for 3 seconds on some models). Alternatively, switch the circuit breaker OFF, wait 30 seconds, and switch it back ON. Allow the unit to complete its startup self-test before calling for hot water again.
Clean the flame sensor rod
schedule10–15 minTurn off gas at the shutoff valve and switch off the circuit breaker. Open the front panel and locate the flame sensor rod — a thin metal rod with a white ceramic insulator near the burner assembly. Gently rub only the rod tip with 0000-grade fine steel wool or 320-grit emery cloth (10–15 light strokes). The metal should look bright and shiny. Reinstall, restore power and gas, and test.
Clean the cold-water inlet filter screen
schedule5–8 minClose the cold-water inlet valve. Slowly open a nearby hot-water tap to relieve pressure. Unscrew the cold-water inlet union fitting and extract the small mesh filter screen. Rinse it thoroughly under running water to remove white scale or brown sediment. Reinstall, re-open the supply valve, and test for ignition.
Inspect the venting system — both intake and exhaust
schedule5–10 minGo outside and check both the exhaust outlet and the air intake terminal. Look for debris buildup, insect nests, ice dams (in winter), or bird nest material blocking the opening. Confirm the vent pipe slopes correctly (per your model's installation manual) and that no sections are crushed, kinked, or have backed out of their joints.
Verify minimum water flow
schedule2 minOpen two hot-water fixtures simultaneously — a shower and a sink, for example — to push total flow above the 0.5 GPM threshold. If the unit fires successfully with both open but not with just one, the single fixture's flow rate is too low. Check the aerator or showerhead for a built-in flow restrictor.
Test the igniter electrode
schedule10–15 minWith the unit de-energized, locate the igniter electrode — a dual-wire component with a ceramic insulator, distinct from the single-wire flame sensor rod. Disconnect the lead wires. Set your multimeter to the resistance (Ω) setting and measure across the electrode terminals. Functional range: 10–30 kΩ. Use a feeler gauge to check the spark gap: target 3.0–4.0 mm. A gap wider than 4.5 mm produces an unreliable spark.
Still seeing code 11? Time to call a professional
schedule—If you have completed all seven steps above and code 11 persists, the fault is inside the sealed system: the gas valve solenoid may not be opening under the PCB's command, or the PCB ignition driver circuit itself has failed. Both require a licensed technician with a manometer, multimeter, and gas-certified tools.
Video Guide: Fixing Rheem Tankless Error Code 11
Prefer to watch? This step-by-step video walks through the most common no-ignition fixes for Rheem tankless water heaters — covering the flame sensor rod cleaning and the gas supply check in detail.
“How to Fix Rheem Tankless Water Heater Error Code 11 — No Ignition Step by Step”
Fix — Which Part to Replace (and When)
Always clean before you replace. Most code 11 cases resolve with cleaning alone. Use this table only after completing the step-by-step diagnosis above.
Flame sensor rod assembly
DIYReplace when: Ceramic insulator is cracked or has carbon tracking, or ionization current stays below 1.0 µA even after cleaning
Cleaning resolves most cases. Replace only when cleaning fails after two attempts.
Igniter electrode
DIYReplace when: Multimeter shows open circuit (∞ Ω), gap is > 4.5 mm, or the ceramic has visible cracks
Use OEM part only — aftermarket electrodes may have incorrect gap geometry.
Cold-water inlet filter screen
DIYReplace when: Screen is damaged or holes are corroded through — usually just clean, rarely replace
Replacement screens are inexpensive. Keep one on hand for hard-water installations.
Gas valve solenoid / assembly
Pro onlyReplace when: Solenoid coil resistance is outside 25–60 Ω range, or valve does not open despite correct PCB drive voltage
Gas valve replacement must be performed by a licensed gas fitter.
Control PCB (main board)
Pro onlyReplace when: All mechanical and sensor components test within spec, but the board is not sending drive voltage to the igniter or gas valve during an ignition attempt
Inspect wiring connectors thoroughly before condemning the board — corroded pins are the most common 'PCB failure' cause.
Stop and call a professional if any of these apply
- arrow_rightYou smell gas at any point — evacuate the area and call your gas utility immediately
- arrow_rightCode 11 persists after completing all 7 diagnosis steps in this guide
- arrow_rightThe unit is still under warranty — DIY repairs beyond cleaning may void it
- arrow_rightYou are not comfortable working near gas appliances
Diagnose faster with HeatSync — free
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Frequently Asked Questions
What does Rheem error code 11 mean?expand_more
Rheem error code 11 means ignition failure — also called no ignition. The unit opened the gas valve and fired the igniter up to three times, but the flame sensor rod never confirmed a stable flame. The control board then locked out for safety.
What is the most common cause of Rheem code 11 no ignition?expand_more
A dirty or oxidized flame sensor rod is the single most common cause — responsible for roughly 40–50% of code 11 cases. Cleaning the rod tip with fine steel wool takes under 5 minutes and often resolves the issue immediately.
What part do I need to replace for Rheem error code 11?expand_more
Start with cleaning before replacing anything. Clean the flame sensor rod and inlet filter first. If code 11 persists, test the igniter electrode with a multimeter. A persistent code 11 after all DIY steps usually means the igniter electrode, gas valve solenoid, or control board needs professional replacement.
Can low water pressure cause Rheem error code 11?expand_more
Yes. Rheem tankless units require a minimum 0.5 GPM to activate the burner. If flow is below this threshold — due to a clogged inlet filter, a partially closed valve, or a low-flow fixture — the unit will not fire at all.
How do I reset Rheem error code 11?expand_more
Press the Reset button on the front panel (hold 3 seconds on some models), or switch the circuit breaker off for 30 seconds then back on. If code 11 returns in the next hot-water cycle, the root cause has not been fixed.
Does Rheem error code 11 damage my water heater?expand_more
No. Code 11 is a safety lockout — the unit shut down before any damage occurred. Resolve the ignition issue, reset, and the unit will resume normal operation.