What causes Rheem Tankless Water Heater 13?

A blocked fresh air intake, which SVC 820 calls the primary cause of code 13. Venting that is blocked, not sealed properly, too long, made of unapproved material or fitted with unapproved terminations. A vent termination placed where exhaust gets pulled back into the fresh air intake (recirculation). Not enough combustion air in the room for an indoor heater. Rheem's RTG 74 bulletin calls for 1 square inch of opening per 1,000 BTU of input, up to 200 square inches (about 14.5 by 14.5 inches). A poor or missing ground connection at the control board or the power supply. A dirty or defective second flame rod (FL-2), or a loose flame rod connection. Gas supply problems such as the wrong gas type, low gas pressure, an undersized gas pipe, regulator or flex line, or air in the gas line, which Rheem says to check at maximum BTU. Wrong altitude settings on the control board, or a blower motor problem.

How do I fix Rheem Tankless Water Heater 13?

Go outside and look at the intake and exhaust terminations. Clear away leaves, snow, nests, insects or anything else blocking them, without taking the vent apart. For an indoor heater in a closet or small room, make sure its combustion air openings aren't blocked or closed off, and that the room isn't sealed up. Rheem's bulletin asks for 1 square inch of opening per 1,000 BTU of input, up to 200 square inches. Check for anything that could be pulling exhaust back into the intake, like a new fence, plantings or stored items crowding the vent outlet. If you had the heater installed, moved or re-vented recently, ask the installer to recheck vent length, materials, joints and termination location against the Use and Care manual, since Rheem lists all of these for code 13. Don't keep restarting the heater to get past code 13. After five shutdowns in four hours it locks out and needs a reset at the control board, which is a job for a technician with the cover off. If anyone in the house feels dizzy, gets a headache or feels sick, or a CO alarm sounds, get everyone outside and call for help. If you smell gas, leave and call your gas company from outside.

When should I call a pro for Rheem Tankless Water Heater 13?

Code 13 returns after the vent terminations are clear and the room has its combustion air openings. The heater has locked out (five code 13 shutdowns within four hours) and needs the control board reset procedure, which SVC 820 says is only for hard lockouts. Inspecting the full intake and exhaust run for blockage, separated or unsealed joints, excess length and recirculation at the termination. Checking the gas supply at maximum firing rate: gas type, pressure, pipe and flex line sizing, regulator and air in the line. Checking the green ground wires at the control board and the ground at the power supply. Checking that the flame touches both flame rods through the sight glass, reseating connectors M and T, reading flame rod status in Maintenance Mode, and cleaning or replacing the FL-2 flame rod. Verifying altitude DIP switch settings and checking the blower motor. The heater is under warranty. Contact Rheem technical support or a Rheem-authorized technician instead of opening it yourself.

Code: 13🚿 Rheem Tankless Water Heater

Rheem Tankless Water Heater Code 13: Poor or Improper Combustion (Meaning, Causes, Fixes)

Q: What does Rheem Tankless Water Heater 13 mean?

Code 13 on a Rheem tankless water heater means the heater is burning, but not burning cleanly. On indoor units a second flame rod watches combustion quality, and code 13 is set when it reads poor or improper combustion. Rheem's troubleshooting manual says the primary cause is a blocked fresh air intake, followed by venting problems, a poor ground and a dirty or failed second flame rod. Check that the intake and exhaust outside are clear and that an indoor heater has enough fresh air, then get a qualified technician if 13 comes back, because a heater that isn't burning cleanly can produce carbon monoxide. Rheem's Tankless Gas Troubleshooting Manual SVC 820 (Mid and High Efficiency platform) titles code 13 "Flame Rod FL-2: Reads Poor Or Improper Combustion" and marks it Indoor ONLY. Indoor models have two flame rods: one confirms there is a flame, and the second (FL-2, on control board connector T) monitors combustion efficiency. The manual's sequence of operations explains that when the air intake is blocked or oxygen isn't sufficient, the flame sensor's output changes; the control board then adjusts the fan and gas valve to prevent imperfect combustion, and if it can't correct the condition the heater locks out with an error code. Warning code 05 is the early version of the same problem: the unit keeps running but may eventually shut down with 11, 12 or 13. Rheem's older RTG 74 fault code bulletin lists 13 as "Oxygen Depletion Sensor (Poor or Improper Combustion)". Code 13 is a hard lockout: SVC 820 says that if the heater shuts down five times within four hours on code 13, it must be reset with the reset procedure on the control board before it will run again. Code 13 doesn't appear in the code tables of Rheem's newer High and Super High Efficiency (condensing) or IKONIC service manuals, so it's mostly seen on Rheem and Ruud RTG-series indoor units.

Reviewed 2026-10-23 · Based on 3 manufacturer & service sources

What the code means

Rheem's Tankless Gas Troubleshooting Manual SVC 820 (Mid and High Efficiency platform) titles code 13 "Flame Rod FL-2: Reads Poor Or Improper Combustion" and marks it Indoor ONLY. Indoor models have two flame rods: one confirms there is a flame, and the second (FL-2, on control board connector T) monitors combustion efficiency. The manual's sequence of operations explains that when the air intake is blocked or oxygen isn't sufficient, the flame sensor's output changes; the control board then adjusts the fan and gas valve to prevent imperfect combustion, and if it can't correct the condition the heater locks out with an error code. Warning code 05 is the early version of the same problem: the unit keeps running but may eventually shut down with 11, 12 or 13. Rheem's older RTG 74 fault code bulletin lists 13 as "Oxygen Depletion Sensor (Poor or Improper Combustion)". Code 13 is a hard lockout: SVC 820 says that if the heater shuts down five times within four hours on code 13, it must be reset with the reset procedure on the control board before it will run again. Code 13 doesn't appear in the code tables of Rheem's newer High and Super High Efficiency (condensing) or IKONIC service manuals, so it's mostly seen on Rheem and Ruud RTG-series indoor units.

Likely causes

A blocked fresh air intake, which SVC 820 calls the primary cause of code 13
Venting that is blocked, not sealed properly, too long, made of unapproved material or fitted with unapproved terminations
A vent termination placed where exhaust gets pulled back into the fresh air intake (recirculation)
Not enough combustion air in the room for an indoor heater. Rheem's RTG 74 bulletin calls for 1 square inch of opening per 1,000 BTU of input, up to 200 square inches (about 14.5 by 14.5 inches).
A poor or missing ground connection at the control board or the power supply
A dirty or defective second flame rod (FL-2), or a loose flame rod connection
Gas supply problems such as the wrong gas type, low gas pressure, an undersized gas pipe, regulator or flex line, or air in the gas line, which Rheem says to check at maximum BTU
Wrong altitude settings on the control board, or a blower motor problem

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Go outside and look at the intake and exhaust terminations. Clear away leaves, snow, nests, insects or anything else blocking them, without taking the vent apart.
For an indoor heater in a closet or small room, make sure its combustion air openings aren't blocked or closed off, and that the room isn't sealed up. Rheem's bulletin asks for 1 square inch of opening per 1,000 BTU of input, up to 200 square inches.
Check for anything that could be pulling exhaust back into the intake, like a new fence, plantings or stored items crowding the vent outlet.
If you had the heater installed, moved or re-vented recently, ask the installer to recheck vent length, materials, joints and termination location against the Use and Care manual, since Rheem lists all of these for code 13.
Don't keep restarting the heater to get past code 13. After five shutdowns in four hours it locks out and needs a reset at the control board, which is a job for a technician with the cover off.
If anyone in the house feels dizzy, gets a headache or feels sick, or a CO alarm sounds, get everyone outside and call for help. If you smell gas, leave and call your gas company from outside.

🛠 When to call a pro

Code 13 returns after the vent terminations are clear and the room has its combustion air openings.
The heater has locked out (five code 13 shutdowns within four hours) and needs the control board reset procedure, which SVC 820 says is only for hard lockouts.
Inspecting the full intake and exhaust run for blockage, separated or unsealed joints, excess length and recirculation at the termination.
Checking the gas supply at maximum firing rate: gas type, pressure, pipe and flex line sizing, regulator and air in the line.
Checking the green ground wires at the control board and the ground at the power supply.
Checking that the flame touches both flame rods through the sight glass, reseating connectors M and T, reading flame rod status in Maintenance Mode, and cleaning or replacing the FL-2 flame rod.
Verifying altitude DIP switch settings and checking the blower motor.
The heater is under warranty. Contact Rheem technical support or a Rheem-authorized technician instead of opening it yourself.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Plug-in carbon monoxide alarm with digital display

Worth having near any gas water heater, especially one throwing a combustion code.

[affiliate link slot: AFFILIATE_SLOT_CO_DETECTOR]

⚙

Combustible gas leak detector

For peace of mind around a gas water heater. Any real gas smell means leave and call the gas company, not a DIY check.

[affiliate link slot: AFFILIATE_SLOT_GAS_LEAK_DETECTOR]

⚙

Rheem tankless flame rod (exact part for your model)

Technician part only. Code 13 points at the second flame rod (FL-2) on indoor models.

[affiliate link slot: AFFILIATE_SLOT_TANKLESS_IGNITER]

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Rheem Tankless Water Heater Code 13: Poor or Improper Combustion (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Plug-in carbon monoxide alarm with digital display

Combustible gas leak detector

Rheem tankless flame rod (exact part for your model)

Related codes

Sources