Baxi Tankless Troubleshooting: Codes & Lockouts

The Short Answer: A Baxi on the wall is a condensing combi, not a 40-gallon and not a Rinnai. Cold showers are E133 ignition, E118 / E119 low CH pressure, E125 no circulation, E160 fan, a seized diverter, or a scaled plate. Photograph the code. One reset after you fix the cause. Do not jumper the low-water cutoff. Hire a licensed tech.

Search Intent & The Real Problem

The wall unit says Baxi, Luna, or Luna Duo-tec. Radiators may still get warm. The kitchen does not. Or the display is flipping E1 then 33. North American Lunas and UK Duo-tec / Platinum / 600 / 800 cousins share the same family of faults: a sealed CH loop that must sit in the green band, a DHW plate and diverter, a 120-volt fan, and a gas train that lockouts if the condensate trap ices. People paste Rinnai 11/12 onto this board. Wrong brand, wrong reset, wrong part.

Sister hydronic: Buderus indirect, HTP Phoenix, QuietSide combi. Mid-shower swing on a dedicated tankless: hot to cold. This page is Baxi / Luna combi DHW.

What other guides get wrong

They treat a Baxi like a tankless with two isolation valves and no heating loop, so a dead CH pump gets a new gas valve. They tell you to keep resetting E133 into a flooded burner or a frozen condensate pipe. They jump the LWCO “to see if it fires.” And they skip the DHW strainer and the diverter, which is the $0 diagnosis when the house is toasty and the shower is ice.

Step-by-Step Troubleshooting

Step 1: Read the plate — Luna combi vs a box that is only DHW

• What to look for: Model, fuel (NG or LP), BTU, 120 V. Photo of the plate and the code before you clear it. CH gauge in the green band cold — often about 12–15 psi / 0.8–1.0 bar. Floor-on-the-gauge is a leak, not a “boost.” House domestic static over ~80 psi will dump a T&P; it does not write E133.
• Required tools: Flashlight, phone camera.

Manifold gas pressure is licensed (about 3.5–7 in. w.c. natural gas / 10–11 in. w.c. LP — confirm the rating plate). Do not loosen fittings to “see if it has gas.”

Step 2: Power, CH pressure, and whether the fan even woke up

• What to look for: 120 V disconnect on. Display dark = power, fuse, or a dead board. Temperature shows but nothing fires = no call, summer / DHW-only mode off, or a lockout that needs the code pulled. Open a hot faucet. If the fan never spins, you do not have a “diverter” problem yet — blower.
• Required tools: Eyes on the disconnect and the CH gauge. A known-good lamp on that circuit. Do not work it live under the cover.

A silent fan plus a call for heat is carbon monoxide if you bypass the prove switch. Do not.

Step 3: Decode Baxi numbers — not a Rinnai sticker

Confirm against your Luna / Duo-tec / 600 sheet. Displays often alternate E1 then 33 for E133.

• What to look for: The number on that display, twice. One restart after you fix the cause — not a reset race. Winter E133 with a plastic condensate run outdoors is ice at the termination until proven otherwise. Warm the pipe from outside with warm water, not a torch, then one reset. If it returns, licensed.
• Required tools: The paper in the jacket. No jumper wires.

E133 is gas and air. You may confirm the appliance gas cock is open and the house has other gas appliances lighting. You may not crack flares. Do not purge a gas line yourself. Closet odor now: leave.

Step 4: DHW path — strainer, turbine, diverter, plate

• What to look for: Unit runs, radiators get warm, faucets stay cold: diverter stuck on heat, DHW turbine never sees flow, or a packed domestic strainer. Unit “runs normally” and still lukewarm: scaled plate exchanger. Isolation valves half shut. Crossover at a mixing cartridge (close cold to the heater, open a hot faucet — pressurized flow means a valve is dumping cold into hot).
• Required tools: Faucet, flashlight on the DHW strainer with power and gas off. Paper towel for cabinet leaks.

A packed strainer is fair DIY with the unit isolated. A diverter, plate, or gas valve is licensed. Scale in a 15-year Luna with no service ports is often a replacement conversation, not a vinegar myth.

Step 5: Condensate, polarity, and replace vs hunt parts

• What to look for: Frozen or disconnected condensate (this is Category IV, not B-vent tape). Exhaust odor: shut down — tankless exhaust smell. Reverse line/neutral on a Baxi makes ignition stupid — licensed electrician, not a new gas valve. Wet PCB after a plate leak stays off. Baxi North America parts are thinner than Rinnai. A licensed tech who will tell you the heat exchanger is a two-week wait is doing you a favor.
• Required tools: Eyes on the flue and the cabinet floor. Disconnect off. If the board is wet, it stays off.

Do not convert LP to NG as a “fix.” Do not keep firing a cracked condensing block.

When DIY Ends: Calling a Licensed Pro

You may photograph the plate and the code, confirm 120 V and the gas cock, read the CH gauge, clean an isolated DHW strainer, and power-cycle once after the cause is fixed. Stop at E133 that returns, a silent fan, gas odor, a wet PCB, a jumpered LWCO, and any CH fill that will not hold. Gas pressure, flame rod, diverter, plate, and venting are licensed. Same-day diagnosis beats a third used board from an auction listing for a brand that left half the channel.

Frequently Asked Questions

Q: Why is my Baxi tankless not producing hot water? A: Low CH pressure (E118 / E119), ignition lockout (E133), a seized pump (E125), a packed DHW strainer, a diverter stuck on space heat, or a scaled plate. Read the code before you buy a part.

Q: What does E133 mean on a Baxi or Luna? A: The board tried to light and did not prove flame — gas supply, electrode, condensate ice, or air after a meter relight. Open the cock fully, one reset after you fix the cause. If E133 returns, stop. Do not keep sparking into a dark burner.

Q: The radiators are hot. Why is the shower still cold? A: The burner is doing space heat. DHW is the diverter, the domestic turbine, the strainer, or the plate. That is not a new gas valve.