Burnham boiler flame rod assembly
Price when cleaning no longer restores stable flame current.
$30-$60
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What Err 4 and Err 6 Mean
A Burnham boiler displaying Err 4 has detected a weak or failing flame signal. The control board is not receiving enough microamps through the flame rod to confirm the burner is lit. Err 6 is the opposite problem. The control board detects a flame signal when the gas valve is closed and the burner should be off.
Both codes point to problems in the flame sensing circuit but require different diagnostic approaches. Err 4 is more common. Err 6 is rarer and often involves a stuck gas valve or electronic noise.
Code Reference Table
| Code | Description | Likely Cause | Severity |
|---|---|---|---|
| Err 4 | Flame current low | Dirty flame rod, weak igniter, bad wiring, no ground path | Medium |
| Err 6 | Flame sensed out of sequence | Stuck gas valve, control board failure, electrical noise | High |
| b08 | Pressure switch open with inducer on | Blocked vent, failed switch, restricted condensate | Medium |
| b09 | Pressure switch closed with inducer off | Stuck switch, shorted wiring | Medium |
| b26 | Safety limit circuit open | High limit tripped, rollout switch, blocked airflow | Medium |
Step-by-Step Diagnosis
Err 4 — Flame Current Low
This is the most common Burnham error on Series 2 and Alpine boilers. The flame sensor rod carries a microamp DC current through the flame to ground. If the rod is dirty or the ground path is poor, current drops below the 1-2 microamp threshold.
Start here:
- Turn off power to the boiler at the service switch and breaker
- Locate the flame sensor rod. It is a metal rod mounted in the burner assembly near the igniter
- Remove the flame rod. Look for white or gray deposits on the rod surface. Carbon or soot buildup reduces signal
- Clean the rod with a fine abrasive pad. Do not use sandpaper. Do not touch the rod with bare skin. Oil from your fingers can cause carbon spotting on the next cycle
- Reinstall the rod. Restore power. Watch the control board during the ignition sequence. If Err 4 clears, the fix was the cleaning
- If Err 4 returns, check the wire from the flame rod to the control board. Look for cracked insulation, loose connections, or corrosion
- Test the ground path. Flame sensing requires a solid ground through the burner assembly. A loose burner ground screw or painted mounting surface can drop signal below threshold
- Check igniter operation. A weak spark igniter can cause flame that is too small for the rod to sense
Err 6 — Flame Without Call
Err 6 is a safety code. The control board will lock out and require a manual reset. This means the board detected a flame signal at a time when the gas valve was commanded closed.
Diagnosis:
- Do not attempt to reset the boiler until you have checked the gas valve. A stuck-open gas valve can release unburned gas into the combustion chamber
- Turn off gas supply to the boiler
- Remove the flame rod connector from the control board. Err 6 should clear if the signal was caused by a failing flame rod. If Err 6 remains with the connector off, suspect the control board
- Check the gas valve for mechanical sticking. Manually cycle the valve. Listen for the solenoid click. If the valve stays open after power is removed, replace the gas valve
- Check for electrical noise on the flame sense wire. The wire can pick up interference from high-voltage ignition cables. Reroute the wire away from spark cables. Replace damaged wire shielding
- If gas valve and wiring check out, replace the control board
Common Parts and Replacement Costs
| Part | Estimated Cost | When to Replace |
|---|---|---|
| Flame rod assembly | $30-60 | Every 2-3 seasons or when cleaning no longer restores signal |
| Igniter | $40-90 | When Err 4 persists with clean rod and good ground |
| Gas valve | $150-400 | When Err 6 is confirmed with stuck valve |
| Control board (UCB-2 or equivalent) | $200-500 | When all other causes are ruled out |
Diagnostic Tools You Will Need
| Tool | Use |
|---|---|
| Fine abrasive pad | Cleaning flame rod |
| Microamp meter | Testing flame current (target: 2-8 microamps DC) |
| Multimeter | Testing gas valve solenoid continuity and ground path |
| Screwdriver set | Removing burner access panels |
When to Call a Professional
Call a licensed boiler technician if any of these apply:
- Err 6 returns after the gas valve is replaced
- You smell gas at any point during diagnosis
- The boiler has multiple lockout cycles per day
- You are not comfortable working with gas controls
- The burner assembly needs removal for flame rod access
Safety note: Err 6 can indicate a gas valve that is leaking through the seat. If you suspect a stuck-open gas valve, shut off the gas supply immediately and call a professional.
Related Guides
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- Boiler condensate blockage troubleshooting