Board repair · fault patterns
Rooftop Unit or Furnace Locked Out
A lockout is the board doing its job. It attempted ignition, failed to confirm flame the specified number of times, and shut the gas off rather than keep trying — which is the behaviour you want from it. So the board starts as the messenger rather than the suspect, and the first move is to read what it is telling you. Almost every board of this class flashes or displays a fault code, and that code is better diagnostic information than anything measurable from the outside.
$85
Bench diagnosis
Credited when the repair is approved
$1,200–8,000
Typical replacement
What the OEM board costs new
10–25%
Repair cost
Of the replacement price
1–3 business days
Turnaround
Bench time, plus transit. 90-day warranty.
Before you pull the board
Cheaper things that produce the same symptom
Work through these first. Each one is faster and cheaper to check than a board is to ship, and each one is a real reason a working board arrives on the bench.
The flash code or display code, before anything else
Count the flashes and look them up for that model. A code for flame sense, pressure switch, limit, or rollout names the circuit to test and saves the entire guessing stage. A board that reports its own internal fault is the one case where the code points back at the board.
The flame sensor — the most likely answer by a wide margin
Flame rectification works in microamps, so a film of oxide on the rod drops the signal below the threshold. Clean it with an abrasive pad, not sandpaper, and measure the current in microamps against the manufacturer figure. This is the fault behind most lockouts and it costs nothing to rule out.
Igniter, gas pressure, and the pressure switch
A cracked igniter that still glows, low manifold pressure, or a pressure switch held open by a blocked flue or a failed inducer all produce a failed ignition the board can only report as a lockout. Check manifold pressure and prove the pressure switch closes on a call.
Limits, rollout switches and condensate
A limit opening on high temperature or a rollout tripping is a safety working correctly, and a blocked condensate drain will lock out units that monitor it. None of these are board faults and all of them will lock out a brand new board.
Then it is the board
What actually implicates it
With the list above cleared, these are the signs that point at the board itself. None of them is proof — that is what the diagnosis is for — but together they are enough to make shipping it worthwhile.
- Flame current measured good in microamps at the rod, and the board still reports no flame
- It locks out without attempting ignition at all — no inducer, no igniter, no gas valve
- The flash code indicates an internal or self-check fault rather than an external circuit
- It will not reset, or resets and locks out again instantly with every external circuit proved
- The gas valve or igniter output measures nothing with the board commanding the sequence
- The lockout follows a storm or an outage, or only happens once the cabinet is hot
Failure modes
What is physically wrong, and why
The cause matters as much as the fault — a board replaced into the condition that killed it fails again.
The flame-sense circuit on the board
The board has to measure a few microamps of rectified current through a sensing network. A leaky component, a cracked solder joint, or a contaminated board surface leaks more than the signal it is trying to read, so a perfectly good flame reads as none. The result is a lockout with nothing visibly wrong anywhere.
Ignition and gas valve drive circuits
These outputs switch inductive loads and sit closest to the high-current side of the board. A failed driver cannot open the valve or energise the igniter, and the board duly reports a failed ignition it caused itself.
Cracked solder joints around the heavy components
Blower vibration plus daily thermal cycling cracks joints under relays, transformers and connectors. The fault is intermittent and moves when the board is flexed, which is why it locks out on some calls and not others and why it is reproducible on the bench under heat.
Failed supply capacitors, giving a brownout mid-sequence
Ignition is a timed sequence. A rail that sags when the inducer and igniter load it can reset the processor partway through, which the board records as a failed attempt. Three of those and it locks out, with every external component good.
Surge damage on the 24 V control side
A transient through the protection component leaves the board working but degraded, and the circuits that notice first are the sensitive ones — flame sense above all. A unit that has locked out since a storm has usually been told exactly what happened to it.
Mail-in
Getting the board to the bench
Book it in first. An RMA number comes back the same business day, and the board arrives with the paperwork already attached to it.
Ship to
System Voidz — Repair IntakePO Box 33 (USPS)413 Railroad Ave (UPS, FedEx)Watrous, NM 87753Use the PO Box for USPS and the street address for UPS or FedEx — a PO Box cannot accept courier deliveries.
Wait for the RMA number before shipping.
Tell us this much on the form
- Which of the checks above you have already cleared. It is the difference between starting the diagnosis and repeating your work.
- Photograph the board in place, connectors attached. It is the single most useful thing you can send.
- The equipment make and model, and the number printed on the board itself.
- Pack it in an anti-static bag. Bubble wrap alone is not enough.
Questions
Locked out after ignition attempts — common questions
How do I know it is the board and not the flame sensor?
Measure the flame current in microamps with the burner lit, at the rod. If it meets the manufacturer figure and the board still reports no flame, the sensing circuit on the board is reading it wrong and that is a board fault.
If the current is low, clean the rod and measure again. Most lockouts end there, which is why it is the first thing to rule out rather than the last.
It locks out intermittently. Is that still worth sending?
Yes, and say so on the form. Intermittent lockouts with good external circuits are usually a cracked joint or a sagging rail, and both are reproducible on the bench under heat and load in a way they never are on a roof at seven in the morning.
Note whether it correlates with outside temperature or with how long the unit has been running. That detail narrows it to one of the two.
The board is under OEM warranty. Should I still send it?
If it is genuinely under warranty, use the warranty — that is the cheaper route and it is what it is for.
Repair is the better answer once the warranty has expired, when the board is discontinued, or when the replacement is quoted on a lead time you cannot wait out.
Equipment
Where this pattern turns up most
The same fault, on the equipment it is most often seen on. Each page covers what that class of board does and what else goes wrong with it.