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Quick answer from a licensed plumber: Rheem and Ruud gas tankless heaters use two styles of code. The RTG, RTGH, RUTG and RUTGH “-3” series show two-character codes: 11 (flame not detected), 12 (flame lost), 10, 90 and 99 (air intake or venting), 29 (condensate not draining), 1L and 88 (flush the heat exchanger) and P1 (not enough water flow).
Rheem’s ultra-low-NOx condensing units (Use and Care Manual AP22745) show A-prefixed codes such as A011 (gas shut-off valve not fully open), A012 (gas service interrupted or propane low) and A090 (air intake or vent blocked). The fixes an owner can make are outside the cabinet: open the gas valve, refill the propane, clear the vent terminals, clean the inlet water filter and faucet aerators, and check the condensate line. Everything behind the front cover belongs to a qualified service technician.
Safety first: if you smell gas, get everyone out, do not use switches, phones or anything that could spark near the heater, and call your gas utility from outside. That is standard gas-appliance safety. Rheem’s manual adds: turn off the manual gas shut-off valve if the heater has been through overheating, fire, flood or physical damage, or if the gas supply fails to shut off, and do not run the heater if the vent shows signs of leaking exhaust.
Which Code List Is Yours?
Rheem’s High and Super High Efficiency troubleshooting manual (AP21822) covers the “-3” series: the non-condensing RTG and Ruud RUTG models (70, 84 and 95) and the condensing RTGH and Ruud RUTGH models (68, 84, 90 and 95), plus RMTG, RMTGH, ECO and ECOH variants. Those show two-character codes. The Use and Care Manual for Rheem’s ultra-low-NOx condensing tankless (AP22745) uses A- and T-prefixed codes. The model number is on the rating label.
RTG, RTGH, RUTG and RUTGH Codes
| Code | Rheem’s description | Owner check, then who |
|---|---|---|
| P1 | Not enough water flow for operation | Needs at least 0.4 GPM: open the tap further, clean aerators and the inlet water filter, check for a crossover. |
| 1L | Lime build-up detected in heat exchanger | Drain and flush. The reset is on the control board: technician. |
| 03 | Manifold setting failure (multi-unit systems only) | Technician. |
| 05 / 28 | Low heat exchanger temperature detected | Technician. |
| 10 | Incorrect inlet air volume | Clear the intake and exhaust terminals outside. |
| 11 | Flame not detected by flame rod | Gas supply valve open; propane in the tank. Then technician. |
| 12 | Detected presence of flame, then lost it | Same gas checks as 11. Then technician. |
| 14 | Over-heat limiter is active | Technician. Rheem says it is normally caused by gas supply or venting. |
| 15, 16 | Excessively hot water detected / outlet thermistor failure | Technician. |
| 17 | Water leak detected (WiFi models) | Look for water; Rheem notes condensation can trigger it too. |
| 24 | DIP switch malfunction: buttons held more than 20 seconds | Stop holding the buttons; if it returns, technician. |
| 29 | Condensation not drained from collector | Check the condensate line for kinks and clogs. |
| 31–35 | Inlet, heat exchanger, outlet and air thermistor faults; improper thermistor connection | Technician. |
| 51, 52, 71, 72, 80 | Gas solenoid and gas valve faults; flame sensed when it should not be | Technician. |
| 61, 79 | Blower motor speed or current | Technician. |
| 66, 70, 76, 82 | Bypass motor; microcomputer; display communication; gas chip | Technician. |
| 88 | Maintenance notice | Flush, then clear the code (steps below). |
| 89 | High exhaust temperature detected | Technician; Rheem again points to gas supply or venting. |
| 90 / 99 | Blockage in venting detected (90 before ignition, 99 after shutdown) | Clear the vent terminals; the lockout reset is technician work. |
Condensing Models With A-Codes
From the Service Error Code Chart in Rheem’s condensing Use and Care Manual (AP22745):
| Code | Possible cause (Rheem) | Rheem’s solution |
|---|---|---|
| A011 | The gas shut-off valve is not fully opened | Check the shut-off valve and open it completely. |
| A012 | Gas service interrupted, or LP gas running low | Contact your gas utility, or refill or replace the LP container. |
| A090 | Air intake or vent exhaust opening may be blocked, or vent pipes not connected properly | Remove any blockage (the air intake needs 12 in / 30 cm of clearance); otherwise a dealer or qualified technician. |
| T113 | Not enough hot water to operate the unit | Increase the water flow from the fixtures. |
| T111 | Reminder to flush the heat exchanger and/or replace the water treatment filter | Contact a dealer or qualified service technician. |
| A01L | Buildup of lime deposits | Contact a dealer or qualified service technician. |
| A014, A029 | Overheating; blocked condensate line | Contact a dealer or qualified service technician. |
| A031–A036 | Inlet, heat exchanger, outlet and exhaust thermistor faults; improper thermistor connection | Contact a dealer or qualified service technician. |
| A055, A061, A063, A065, A066 | Gas valve; blower motor; built-in recirculation pump (“SR” models); water flow and bypass solenoids | Contact a dealer or qualified service technician. |
| A070, A071, A072, A082 | Control board; gas solenoid valve; flame detected before ignition; software error | Contact a dealer or qualified service technician. |
To clear an A-code, Rheem’s steps are: turn off all hot water faucets, press SERVICE, press ENTER, scroll with the arrows to “Clear Alarms?”, press ENTER, choose “Yes” and press ENTER. If the code is still shown, turn off the faucet and the heater, note the code and call for service. Rheem also says that if the code is not in the chart at all, turn the heater off immediately and call for service.
I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.
11 and 12: The Flame Codes
Rheem says the heater tries to ignite twice before it shows 11, and shows 12 when it loses the flame three times in a row during a run. Its check list for 11 is gas supply pressure, the gas filter, the venting, the igniter rod and the flame rod. Your part: make sure the gas supply valve to the heater is fully open, that propane is not running low, and that the vent terminals outside are clear. Gas supply pressure has its own specification in Rheem’s technician manual, and cleaning the flame rod is part of a technician’s flowchart. Those belong to a qualified technician. Choosing an LP unit? See our propane tankless water heater picks.
10, 90 and 99: Air and Venting
Rheem says 10 appears when the heater was run before the vent was installed or the blower is not moving enough air, and 90 and 99 when it senses a blockage in the venting before ignition or after shutdown. Go outside and look at both the intake and exhaust terminals: snow, leaves or a nest.
Clear what you safely can. (Rheem’s condensing Use and Care Manual asks for 12 inches of clearance at the air intake; check your own manual for your model.) Codes 10, 13 and 99 are what Rheem calls hard lockouts, and its reset involves removing the front cover and flipping DIP switches on the control board with the unit plugged in. That is technician work: Rheem’s condensing Use and Care Manual warns that removing the front cover panel exposes you to live electricity.
29: Condensate
On models with a built-in condensate drain trap, 29 means the sensor in it found the trap full or could not read the water level. Rheem’s checks: on a new install, the protection plug on the drain connection may not have been removed; the drain line should not be blocked or pinched; and it does not need a P-trap. You can follow the line and look for kinks or a clog. Rheem’s condensing Use and Care Manual also says the condensate is acidic and an external neutralizer needs to be installed to local codes; if one is installed, Rheem says to check periodically that its medium is not depleted.
P1, No Code and Not Enough Hot Water
Rheem says the heater needs at least 0.4 GPM to fire. If it shows P1, or nothing at all and no hot water:
- Open the hot tap further, and clean the aerator on the faucet you are testing. See how to clean a faucet aerator.
- Clean the inlet water filter. Rheem’s condensing manual says to do it monthly: turn the heater off and unplug it, turn off the water supply, drain the heater, unscrew the filter from the base of the cold water inlet, rinse it under running water (a soft brush for heavy sediment), and screw it back in snug. Do not tap, force or overtighten it; Rheem says that can deform the filter.
- Check for a plumbing crossover, using the test in Rheem’s technician manual: turn off the hot water valve at the heater, then turn on only the hot side at each fixture. If water flows freely, hot and cold are crossed somewhere (Rheem notes that single-handle pressure-balancing valves will not let any water flow during this test if there is a crossover), and finding it is a job for a licensed plumber.
- On a new installation, make sure the hot and cold lines are not reversed at the heater.
1L and 88: Flush the Heat Exchanger
1L means the control board has detected lime inside the heat exchanger, and Rheem says the unit must be drained and flushed to prevent permanent damage. 88 is the maintenance notice: Rheem calls it a warning code and says the unit will eventually shut itself down, so flush it rather than just clearing the code. Rheem’s flush, from the troubleshooting manual:
- The gas must stay OFF for the whole flush, and both water supplies are turned off at the service valves.
- Hoses go on the hot and cold service valves under the heater, into a 5-gallon bucket.
- Use 2 gallons of virgin food-grade white vinegar, circulated with a submersible utility pump for 45 to 60 minutes.
- Drain, clean the inlet water filter, then run fresh cold water through for about 5 minutes to rinse.
- Rheem names its own kit for this, the Rheem/Ruud Tankless Flush Kit RTG20124.
To clear 88 after the flush, Rheem’s steps are: turn on the remote control, close the gas and water shut-off valves, press the DOWN button until the lowest temperature shows, press and hold DOWN and UP together for 5 to 10 seconds, then reopen the valves. Repeat if 88 is still shown. The 1L reset is different: it uses the MIN and MAX buttons on the control board inside the cabinet, which is technician territory.
Worth knowing: Rheem’s Use and Care Manual lists flushing under “Professional Preventive Maintenance,” done annually or as a professional recommends, and says certain water conditions can cause damage the limited warranty does not cover. Flushing needs isolation valves with service ports; if yours has none, adding them is a job for a licensed plumber.
The Flush Kit, If You Flush It Yourself
Superior Pump 91660 Tankless Water Heater Descaler Pump Kit. The listing says it includes a 1/6 HP pump, a 3.5-gallon pail with a screw-top lid, two 3/4-inch garden-hose-thread (GHT) stainless steel hoses 6 feet long and a 7 oz packet of descaler powder, and that it is designed to clean and descale most tankless water heaters. Rheem’s steps use a 5-gallon bucket with a submersible utility pump set in the bottom of it; the kit’s pail is 3.5 gallons, so use a 5-gallon bucket as Rheem does.
Rheem’s own RTG20124 kit was down to its last few units when we checked, so we did not link it. Rheem’s procedure specifies vinegar, so use vinegar rather than the included powder unless your manual says otherwise. It was shipped and sold by Amazon.com, new and in stock, when we checked on September 24, 2026; details come from the Amazon listing, not from hands-on testing.
For the full walk-through, including the isolation valves you need before any of this works, see how to descale a tankless water heater, and for the other kits we looked at, the best water heater flush kits.
When to Stop and Call a Licensed Professional
- You smell gas, or the vent shows signs of leaking exhaust.
- Any code whose fix is inside the cabinet: gas valve and solenoids, sensors, blower, control board, flame rod.
- A hard lockout (10, 13 or 99) that needs the control-board reset.
- Code 14 or 89, which Rheem ties to gas supply or venting.
- The relief valve discharges from time to time: Rheem says this may indicate a problem in the water system, to contact the water supplier or a plumbing contractor, and never to plug the relief valve outlet.
- The heater has been through a flood, fire, overheating or physical damage.
FAQ
What does error code 11 mean on a Rheem tankless water heater?
On Rheem and Ruud “-3” series units, code 11 means the flame was not detected by the flame rod. Rheem says the heater tries to ignite twice before it shows 11. Check that the gas supply valve is open and, on propane, that the tank is not empty; gas pressure, the gas filter, venting, the igniter and the flame rod are for a qualified technician.
Are Ruud tankless error codes the same as Rheem?
For the models in Rheem’s troubleshooting manual, yes. That manual covers Rheem RTG and RTGH models and Ruud RUTG and RUTGH models together, with one error code list for all of them.
How do I clear an error code on a Rheem tankless?
On the condensing models with A-prefixed codes, Rheem’s steps are: turn off all hot water faucets, press SERVICE, press ENTER, scroll to “Clear Alarms?”, press ENTER, choose Yes and press ENTER. If the code is still shown, turn the heater off, note the code and call for service. Code 88 on the older series has its own clearing steps, done after the flush.
What does 1L mean on a Rheem tankless?
The control board has detected lime build-up inside the heat exchanger. Rheem says the unit must be drained and flushed to prevent permanent damage. The 1L reset is done with buttons on the control board inside the cabinet, so leave that part to a qualified service technician.
Why does my Rheem tankless show P1?
P1 means there is not enough water flow for the heater to run. Rheem says it needs at least 0.4 gallons per minute to activate. Open the tap further, clean the faucet aerator and the heater’s inlet water filter, and check for a plumbing crossover.
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Plumbing Picks is written by a Georgia journeyman plumber, and the license can be checked through the Georgia Secretary of State’s license verification portal. Every code meaning and step on this page comes from the manufacturer documents listed below, which we opened and read; where we add our own advice, the page says so. Product details come from the Amazon listing. Where a link goes to Amazon we may earn a commission at no extra cost to you. This page is general information for owners, not an offer of repair work. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- Rheem: High and Super High Efficiency Tankless Water Heater Troubleshooting Manual (AP21822 Rev 02) — models RTG, RTGH, RUTG, RUTGH, RMTG, RMTGH, ECO and ECOH “-3” series; error code list and diagnostics
- Rheem: Condensing Tankless Water Heater Use and Care Manual (AP22745 Rev 04) — Care and Cleaning, Troubleshooting Chart, Service Error Code Chart (pp. 82–93)
Bottom Line
Match your model to the right code list first. For 11, 12, A011 and A012, confirm the gas is on and the propane tank is not empty; for 10, 90, 99 and A090, clear the vent terminals; for P1 and T113, get more flow and clean the filters; for 1L, 88 and T111, the heat exchanger is due for flushing (Rheem sends T111 to a dealer or technician). Anything behind the front cover, and any hard lockout, is a job for a qualified service technician.