Burnham Boiler B26, Err 4, Err 6 — Safety and Flame Sensing Faults
Three of the most common Burnham boiler fault codes involve either a safety limit device that tripped or a flame sensing problem. B26 points to a manual-reset safety limit that opened. Err 4 means the flame rod signal fell below the minimum current threshold. Err 6 means the control board detected a flame signal when no flame should be present.
These codes appear on Burnham Alpine, Series 2, MPO, Pinnacle, ES2, and SCG boilers. The control board type determines what the boiler actually displays. UCB-1 boards (older models) use single-digit codes. UCB-2 boards (Alpine, Series 2) use the “Err” format. KCB boards (ES2, SCG) use fault numbers. The B26 code is specific to controls that display two-digit fault codes, typically on UCB-2 and KCB platforms.
| Control Board | Used On | Code Format |
|---|---|---|
| UCB-1 | Older Burnham boilers (pre-2010) | Single digit codes (1-9) |
| UCB-2 | Alpine, Series 2, MPO, Pinnacle | Err 1 through Err 9, plus B-codes |
| KCB | ES2, SCG | Fault numbers including B26 |
B26 — Safety Limit Open
What it means: A manual-reset safety device in the burner circuit has opened. This could be the high-limit aquastat, the rollout switch on the burner vestibule panel, the blocked vent switch on the vent pipe, or a cascading safety relay.
B26 Common Causes
| Cause | Details |
|---|---|
| High limit tripped | Water temperature exceeded the high-limit setpoint (usually 200-210°F) |
| Rollout switch tripped | Flame rolled out of the combustion chamber instead of staying contained |
| Blocked vent switch opened | Flue gas flow is restricted. Vent pipe blocked, condensate backup, or wind |
| Flame roll-out event | Heat exchanger flue passages are plugged (soot, debris, failed baffles) |
| Failed safety device | The limit device itself has failed open |
| Wiring fault | Short or open in the safety limit circuit |
B26 Diagnosis
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Identify which safety device tripped. Trace the manual reset limit circuit. Look for devices with a red or black reset button. Burnham boilers typically have a rollout switch (visible on the burner panel, small red button), a blocked vent switch (mounted on the vent pipe near the boiler top), and the high-limit aquastat (inside the control well, settable temperature dial). Check each one.
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Press the reset button on the tripped device. You will feel a distinct click when it resets. If you find a tripped device and the button resets cleanly, you still need to determine why it tripped.
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For high limit trips: Check system pressure and circulation. If a circulator pump failed or a zone valve is stuck closed, the boiler produces heat faster than the system can remove it. Verify water flow by feeling the supply and return pipes. A cold return pipe with a hot supply means poor circulation.
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For rollout switch trips: Remove the burner access panel and inspect the combustion chamber. Soot buildup on the heat exchanger surfaces or a cracked heat exchanger can cause flame rollout. Check that the burner is properly seated and the gasket seals correctly.
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For blocked vent switch trips: Inspect the vent pipe for obstructions. On condensing models (Alpine, Pinnacle), check the condensate drain for blockages. A backed-up condensate trap fills the vent pipe with water, tripping the blocked vent switch. Clear the drain and flush the trap with water.
B26 Reset Procedure
After correcting the root cause, press the red reset button on the tripped safety device. On some Burnham models, you must also cycle power to the boiler (off/on) to clear the control board fault. If the boiler fires and B26 does not return, the repair is complete. If B26 returns immediately, do not keep resetting. The underlying cause is still present.
Err 4 — Flame Current Low
What it means: The flame rod (flame sensor) detected a flame, but the electrical current passing through the flame to ground fell below the minimum threshold required by the control board. The board interprets this as an unsafe or unreliable flame and shuts down.
Burnham controls require a minimum flame current. Typical thresholds:
| Condition | Flame Current | Result |
|---|---|---|
| Normal operation | 2.0-6.0 microamps (µA) | Stable flame proven |
| Marginal | 1.0-1.9 µA | Intermittent Err 4, nuisance lockouts |
| Below threshold | Below 1.0 µA | Err 4 immediately |
Err 4 Common Causes
- Dirty flame rod. Carbon, oxide, or soot deposits on the flame sensor rod insulate the rod from the flame, reducing current flow. This is the most common cause of Err 4.
- Cracked flame rod insulator. The ceramic insulator around the rod develops a hairline crack. The flame signal leaks to ground before reaching the rod tip.
- Flame rod not in the flame envelope. The rod shifted out of position during burner servicing, or the burner assembly was not reinstalled correctly.
- Weak burner flame. Low gas pressure, a partially blocked burner port, or incorrect air shutter adjustment produces a small, lazy flame that does not fully contact the rod.
- Poor ground path. The burner ground connection is corroded or loose. The flame circuit requires a clean ground through the burner chassis back to the control board.
- Wrong rod gap. On some Burnham models, the flame rod tip must be a specific distance from the burner surface. Incorrect gap reduces signal.
Err 4 Diagnosis and Fix
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Measure the flame current. The UCB-2 control board on Alpine and Series 2 models displays flame current in the diagnostic menu. Navigate to: Menu → Service → Flame Signal. A reading below 2 µA with the burner running means a dirty or failing flame rod.
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Clean the flame rod. Turn off power to the boiler. Remove the flame rod (typically one screw on a bracket near the burner). Clean the rod tip with an emery cloth or fine abrasive pad. Use 400 grit or finer. Do not use steel wool. Steel wool leaves metal fibers on the rod that create false flame paths. Wipe the rod clean with a dry cloth after abrading.
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Inspect the ceramic insulator. Look for cracks, carbon tracking, or glaze damage on the white ceramic insulator. Any visible crack means replace the rod. A cracked insulator will cause intermittent Err 4 that cleaning does not fix.
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Check the burner ground. Verify that the ground wire from the burner to the control board chassis is clean and tight. Use a multimeter to check continuity between the burner surface and the control board ground terminal. Resistance should be near zero ohms.
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Check gas pressure. Measure the manifold gas pressure with a manometer. Natural gas burners need 3.5 inches water column at the manifold. Low pressure produces a weak flame that does not generate enough flame current.
Err 6 — Flame Without Call
What it means: The control board detected a flame signal when there was no call for heat. The burner should be off, but the flame rod is reporting flame. The board interprets this as a gas valve failure and locks out.
Err 6 Common Causes
| Cause | Details |
|---|---|
| Gas valve leak-through | The gas valve seat does not seal completely, allowing gas to seep past and ignite from the hot heat exchanger or pilot flame |
| Flame rod touching ground | The rod is bent, displaced, or dangling against the burner or chassis, creating a false flame signal |
| Control board failure | The board’s detection circuit failed, reading a false flame-on signal |
| Moisture on flame rod | Condensation on the rod during off-cycles creates a conductance path that the board reads as flame |
| Held-over gas | Residual gas in the combustion chamber ignites when the burner cycles off |
Err 6 Diagnosis and Fix
This code is serious. A gas valve that leaks through can allow unburned gas to accumulate. Err 6 is a safety lockout that requires finding the root cause before restarting.
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Inspect the flame rod position. Open the burner access door and check that the flame rod is firmly mounted, not bent, and not touching anything metal. If the rod insulator is cracked or the rod itself is bent, replace it.
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Check the gas valve. If the rod is clean and positioned correctly and Err 6 persists, the gas valve may have a leaking seat. Test by installing a manometer on the downstream side of the gas valve with the boiler off. Any positive pressure reading means gas is passing through the closed valve. Replace the gas valve.
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Check for lingering flame. On some Burnham boilers with hot-surface ignition, the igniter glows hot enough to ignite residual gas after the valve closes. Verify the igniter de-energizes immediately after flame sense is lost. Replace the igniter control module if it stays hot.
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Cycle power and re-test. If the code was triggered by a transient condition (moisture, brief flame rod displacement), cycling boiler power may clear it. If Err 6 returns on the next call for heat, replace the gas valve.
Complete Code Reference
| Code | Meaning | Common Fix |
|---|---|---|
| B26 | Manual-reset safety limit open (hi-limit, rollout switch, blocked vent) | Find tripped device, fix root cause, press red reset |
| Err 1 | Ignition lockout | Check gas supply, igniter, flame sensor |
| Err 2 | Lost flame during operation | Clean flame rod, check gas pressure |
| Err 3 | High limit tripped | Check circulator, purge air, verify system fill |
| Err 4 | Flame current below minimum | Clean or replace flame rod, check ground |
| Err 5 | Low water cutoff | Fill system, check LWCO probe |
| Err 6 | Flame detected without call for heat | Check gas valve, flame rod position |
| Err 7 | Blocked flue or pressure switch | Check vent, condensate drain, inducer |
| Err 8 | Outdoor sensor fault | Check outdoor temperature sensor wiring |
| Err 9 | System sensor fault | Check supply/return temperature sensors |
When to Call a Professional
B26 involves safety limit devices that protect the boiler from fire, explosion, and overheating. If you find a tripped rollout switch or blocked vent switch, the cause must be diagnosed. Do not just reset and walk away. Persistent Err 4 signals a flame rod, gas pressure, or burner tuning issue that requires combustion analysis equipment to diagnose properly.
Err 6 involves the gas valve. A leaking gas valve is a safety hazard. If cleaning the flame rod and checking its position does not resolve Err 6, call a licensed technician. Do not operate the boiler with a confirmed gas valve leak.
Burnham Alpine and MPO boilers require specialized combustion setup (CO2/CO analysis, gas valve offset adjustment) that is beyond basic troubleshooting. For any code that returns repeatedly after basic checks, bring in a qualified service provider.
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