Lennox furnace Alert code 290: Ignition circuit fault — Watchguard
Alert 290 reports a problem in the furnace ignition circuit and puts the system into Watchguard mode. Lennox's instruction is to measure the igniter's resistance against its published specification.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
This is the ignition circuit as a circuit, rather than the igniter as a part. The control energises the hot surface igniter and expects to see a particular current draw; when what it measures does not match, it reports this alert and stops. The most common single explanation is an igniter that has cracked and gone open, which is why Lennox's troubleshooting step is to measure its resistance and replace it if the reading falls outside specification. That the specification lives in each furnace's own installation instructions rather than in this guide is deliberate — the correct range differs by model, so a resistance reading is only meaningful against the right sheet.
Most common causes
- A cracked or failed hot surface igniter, which is open circuit.
- Igniter wiring damaged, loose, or unseated at the connector.
- A failed igniter relay or driver on the control board.
- An igniter that is intact but out of its specified resistance range, and therefore failing.
- Damage from previous service work, since igniters break easily when handled.
How to diagnose it
- Note whether the igniter glows at all during a start attempt, if the furnace has a sight glass.
- Note the age of the furnace and whether the igniter has ever been replaced, since these are consumable parts.
- Note whether the alert began immediately after a service visit.
- Do not measure the igniter yourself — it sits in the burner compartment.
How to fix it
- Technician work in the burner compartment and ignition circuit.
- Ask that the igniter resistance be measured against the range in that furnace's installation instructions, as Lennox specifies, rather than replaced on suspicion.
- A replacement should be the correct part for the model, because resistance ranges differ between them.
- If the igniter measures within range, the fault is upstream in the wiring or on the control board.
Notes
Alert 207 reports the igniter itself sensed open, while 290 reports the ignition circuit as a whole and carries a Watchguard lockout. In practice both frequently end at the same replacement part.
Related
- Alert code 207 — Hot surface igniter sensed open
- Alert code 270 — Flame failed to ignite — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Alert 290 forks into two prices, and the resistance measurement Lennox specifies is the fork in the road. Branch one, far more common: the igniter itself reads open or out of its model-specific range, and the ignitor guide's $150–350 installed applies ($250–500 after hours). Branch two: the igniter measures healthy, which indicts the circuitry that energises it — on the furnace control — and moves the job to board territory at $400–900. The measurement that separates a $200 visit from a $700 one takes a technician about a minute, which is why the wrong move financially is buying an igniter on spec: if the circuit is the fault, the new element changes nothing and its cost is sunk. More codes like this across brands: igniter fault codes.
DIY or call a pro
Where a plain igniter-open code leaves room for a careful like-for-like swap, this alert argues for the technician, and the reason is in its name: circuit fault, not part fault. The control is reporting that what it measured across the whole ignition circuit did not match expectation — element, wiring, connector and triggering circuitry are all inside that statement — so the amateur play of replacing the visible ceramic part is a guess at one suspect among four. The check against the published resistance specification decides it properly, and that specification lives in the model's own installation instructions, read with a multimeter on a disconnected circuit inside the burner compartment. Homeowner contribution: whether the furnace clicks and attempts (then fails) versus never attempting at all, and whether any igniter was replaced recently — a fresh element behind a recurring 290 is itself strong evidence the fault is the triggering side.
The same fault on other brands
Other control families make the same part-versus-circuit split Lennox makes here: the shared Goodman/York board reserves 12 red flashes for ignitor relay failure — the board side — while Trane's E63 covers its igniter circuit. Contrast the brands that cannot see the difference: Goodman's rapid flash family pairs a failed igniter with a bad earth ground in one code because those controls cannot distinguish them. Lennox's 290 sits in the more informative camp, which is exactly why the resistance measurement is the whole diagnosis.
What this looks like
From the outside, this usually shows up as the furnace turns over but never lights — that walkthrough covers what to check in order, and when it stops being a homeowner job.