How House Works

Rheem code 12 (flame lost): what to check

By How House Works EditorialBased on Rheem manualsChecked against sources Updated

Rheem display

12

“Gas service has been interrupted.”

RTGH-3, RTG-3, RTG-R-3, Ruud Mid-Efficiency · RTGH-3 Use and Care Manual, p. 75; RTG-3 Use and Care Manual, p. 77; RTG-R-3 Use and Care Manual, p. 74; RTGH-3 and RTG-3 Troubleshooting Manual, p. 40; RTG-R-3 Troubleshooting Manual, p. 33; Tankless Water Heater Use and Care Manual AP18000-2, p. 26
Owner checks, then technician

Second row in the same charts: "LP gas is running low (LP models only)." Troubleshooting manual name: "Detected Presence of Flame, Then Lost it", shown after the flame is lost 3 times in a row (AP21822 tech manual, p. 40). The EZ installation guides give "Check gas supply." as the first remedy, and the RTGH guide adds a DIP switch row (RTGH EZ guide, p. 7; RTG EZ guide, p. 2; RTG-R EZ guide, p. 2).

What it means

Code 12 means the burner lit and then went out. When the heater loses its flame while burning, it stops the fuel, tries to restart and counts the losses; after 3 in a row it shows 12 RTGH-3 and RTG-3 Troubleshooting Manual, AP21822, p. 40; RTG-R-3 Troubleshooting Manual, AP24838, p. 33. The owner charts list two causes, each with its own remedy: interrupted gas service, and on LP models a propane supply that is running low RTGH-3 Use and Care Manual, p. 75; RTG-3 Use and Care Manual, p. 77; RTG-R-3 Use and Care Manual, p. 74.

Check these first

If a gas leak is present or suspected, do not try to find the cause yourself RTGH-3 manual, p. 5.

  1. Close all hot water taps. Turn the heater off with the POWER ON/OFF button on the remote control, wait about 5 minutes, turn it back on, then open a hot tap and check the display RTGH-3 manual, p. 74; RTG-3 manual, p. 76; RTG-R-3 manual, p. 73.
  2. If 12 is still shown, close the tap, turn the heater off, unplug it for about 30 seconds and plug it back in RTGH-3 manual, p. 74.
  3. If gas service to the house may have been interrupted, contact your gas utility company. That is the chart’s remedy for the first cause RTGH-3 manual, p. 75.
  4. On a propane heater, refill or replace the LP gas container RTGH-3 manual, p. 75. The data plate shows which fuel your heater is built for RTGH-3 manual, p. 5.
  5. Restart the heater, open a hot tap and check the display. If 12 is still shown, turn the heater off, write down the code and call for service RTGH-3 manual, p. 74.

When to call a licensed technician

  • Code 12 comes back while the gas supply is on and the propane tank has fuel.
  • Anything on the troubleshooting manual’s list for 12. Rheem writes that manual for technicians and service professionals only AP21822 tech manual, p. 7. Its checks are the gas supply pressure at the heater’s gas connection, the gas filter at that connection, the vent connections and termination, the flame rod, and DIP switch settings for venting and altitude AP21822 tech manual, p. 40.
  • The manual explains why pressure is measured twice, at standby and at maximum firing: it can be normal at standby and still drop when another gas appliance runs AP21822 tech manual, p. 40. The correct range is 4.0–10.5 in. w.c. for natural gas and 8.0–13.0 in. w.c. for propane AP21822 tech manual, p. 40. If the pressure is too high or too low, the Use and Care manual says to contact your gas supplier for correction RTGH-3 manual, p. 45.
  • Above 2,000 ft (610 m), a qualified service technician makes the altitude adjustments RTGH-3 manual, p. 47.

Notes for different models

The EZ installation guides word the remedies differently. All three give “Check gas supply.” for interrupted gas service and send a low gas supply to a dealer or qualified service technician RTGH Tankless Easy Installation Guide, p. 7; RTG Tankless Easy Installation Guide, p. 2; RTG-R Tankless Easy Installation Guide, p. 2. The RTGH guide adds a row for DIP switch settings RTGH EZ guide, p. 7. Those switches sit on the control board behind the front cover AP21822 tech manual, p. 61, and only a qualified service technician should remove that cover RTGH-3 manual, p. 49.

IKONIC heaters show A012, with the same two causes in their chart IKONIC Use and Care Manual, p. 107. The IKONIC service manual counts 15 flame losses in a row before A012 instead of 3 IKONIC Technical Training and Service Manual, p. 41. The chart for Ruud Tankless Mid-Efficiency heaters prints the same two rows for 12 Tankless Water Heater Use and Care Manual AP18000-2, p. 26.

Related Rheem codes

  • 11 “The gas shut-off valve is not fully opened.”
  • 90 “Air intake or vent exhaust opening may be blocked.”
  • 80 “Detected flame presence after closing solenoid valves”

All Rheem codes

Rheem is a trademark of its owner. How House Works is not affiliated with or endorsed by Rheem.

Sources

  1. Rheem Manufacturing Company. RTGH-3 Use and Care Manual. AP21820 Rev 06, no printed date; cover title: Super High Efficiency Condensing Tankless Water Heater Use and Care Manual; the same file is posted for Ruud RUTGH models; p. 1 is the unnumbered cover. Accessed Oct 12, 2026.
  2. Rheem Manufacturing Company. RTG-3 Use and Care Manual. AP22009 Rev 06, no printed date; cover title: High-Efficiency Non-Condensing Tankless Water Heater Use and Care Manual; the same file is posted for Ruud RUTG models. Accessed Oct 12, 2026.
  3. Rheem Manufacturing Company. RTG-R-3 Use and Care Manual. AP23962 Rev 02, no printed date; cover title: High-Efficiency Non-Condensing Tankless Water Heater with Recirculation Use and Care Manual; the same file is posted for Ruud RUTG-R models. Accessed Oct 12, 2026.
  4. Rheem Manufacturing Company. RTGH Tankless Easy Installation Guide. AP23365, no revision or date printed; no page numbers printed, pages given are PDF pages. Accessed Oct 12, 2026.
  5. Rheem Manufacturing Company. RTG Tankless Easy Installation Guide. AP23366, no revision or date printed; no page numbers printed, p. 2 is the PDF page. Accessed Oct 12, 2026.
  6. Rheem Manufacturing Company. RTG-R Tankless Easy Installation Guide. AP23964 Rev 00, print-file stamp 7/7/25; no page numbers printed, p. 2 is the PDF page. Accessed Oct 12, 2026.
  7. Rheem Manufacturing Company. RTGH-3 and RTG-3 Troubleshooting Manual. AP21822 Rev 02, no issue date printed; cover title: High and Super High Efficiency Tankless Water Heater Troubleshooting Manual; written for technicians and service professionals only (p. 7). Accessed Oct 12, 2026.
  8. Rheem Manufacturing Company. RTG-R-3 Troubleshooting Manual. AP24838 Rev 01, no issue date printed; cover title: High Efficiency Tankless Water Heater with Recirculation Troubleshooting Manual; written for technicians and service professionals only (p. 6). Accessed Oct 12, 2026.
  9. Rheem Manufacturing Company. IKONIC Use and Care Manual. AP22745 Rev 07, no printed date; cover title: Condensing Tankless Water Heater Use and Care Manual; Ruud's S and SR Series pages post Rev 06. Accessed Oct 12, 2026.
  10. Rheem Manufacturing Company. IKONIC Technical Training and Service Manual. AP23209 Rev 03, no printed date; written for technicians and service professionals only (p. 4). Accessed Oct 12, 2026.
  11. Rheem Manufacturing Company (posted on ruud.com). Tankless Water Heater Use and Care Manual AP18000-2. AP18000-2 Rev 03, no printed date; cover title: Tankless Water Heater Use and Care Manual, Noncondensing 199,900 Btu/h, 180,000 Btu/h, and 150,000 Btu/h Models; the manual prints no model numbers and no -3 suffix; ruud.com posts it on its Tankless Mid-Efficiency 70, 84 and 95 product pages (2026-10-12). Accessed Oct 12, 2026.