If your Trane furnace has a red light flashing behind the blower door, first check the rhythm. A slow, steady "heartbeat" blink is normal — it just means the control board has power and is healthy. A counted pattern — the same number of quick flashes, a pause, then the same number again — is a diagnostic code telling you why the furnace shut down. Trane and American Standard furnaces use identical boards, so the same chart applies to both.
Reading the code: ignore the slow heartbeat; count the flashes in a repeating group (four flashes, pause, four flashes is a 4-flash code). Flash-code meanings vary by control-board generation, so always confirm the count against the diagnostic label printed inside your furnace's blower compartment. Newer Trane variable-speed boards drop the blink count and show a 2-digit numeric code instead — read those directly off the board or label.
Trane furnace red-light flash codes
These apply to older and "value" single-stage boards that flash a count:
| Code | Meaning | Typical cause |
|---|---|---|
| Slow "heartbeat" | Normal — board has power, no fault | None; the furnace is healthy and waiting for a call |
| 2 flashes | System lockout (retries / recycles exceeded) | Repeated ignition or flame failures that tripped a lockout |
| 3 flashes | Vent / pressure switch error | Blocked intake/exhaust vent, weak inducer, clogged condensate, or a hose fault |
| 4 flashes | Open high-temperature limit | Dirty filter, restricted airflow/ducts, closed vents, or a weak blower |
| 5 flashes | Flame sensed when none should be | Stuck-open gas valve, or a flame-sensor/board fault |
| 6 flashes | 115V reversed polarity / poor ground | Hot and neutral reversed at the outlet or wiring, or a missing ground |
| 7 flashes ⚠ | Gas valve circuit (some boards: ignition lockout) | Gas-valve wiring/coil — but meaning varies by board; verify on the unit |
| 8 flashes | Low flame-sense signal | Dirty flame sensor, low gas pressure, or poor burner ground |
| 9 flashes ⚠ | Check igniter circuit (some boards: limit open) | Hot-surface igniter or its wiring — but meaning varies by board; verify on the unit |
About the 7 and 9 codes: these two shift meaning between board generations — a 7 is a gas-valve circuit fault on some boards and an ignition lockout on others; a 9 is an igniter-circuit check on some and an open limit on others. We don't guess; we read the code on your specific board and confirm against the diagnostic label inside your unit.
A note on the outdoor unit: if you're looking at a flashing LED on a non-communicating Trane AC or heat-pump board, note that the outdoor flash mapping is not standardized across models — there is no single reliable AC flash chart. Read that unit's own diagnostic label rather than trusting a generic list.
What the codes are telling you
The furnace codes group into familiar families:
- Airflow / limit (4 flashes). The furnace overheated because air isn't moving — nearly always a dirty filter or restricted ducts. The most DIY-friendly code.
- Venting / pressure (3 flashes). The inducer isn't proving a proper draft — a blocked vent, weak inducer, or clogged condensate.
- Ignition / flame (2, 5, 8, and possibly 7/9). Igniter, flame sensor, and gas valve. A cleaned flame sensor clears many weak-flame (8) and lockout (2) codes.
- Electrical (6, and possibly 7/9). Reversed line polarity or a grounding fault.
Common causes
Most common first:
- Dirty air filter or restricted ducts — behind most 4-flash limit trips.
- Dirty flame sensor — the usual reason for an 8-flash weak signal or a 2-flash lockout.
- Blocked vent, weak inducer, or clogged condensate — behind 3-flash pressure faults.
- Reversed polarity or poor ground — the 6-flash code, common after wiring work.
- Failed hot-surface igniter or gas-valve circuit — the 7/9 codes (verify meaning on the unit).
The usual fix
Our tech reads the stored code, then confirms it rather than trusting the number: check and change the filter, clean or replace the flame sensor, verify the inducer and venting, test the limit and pressure switches, check line polarity and ground on a 6, and inspect the igniter and gas valve on a 7/9. Many flashing-Trane calls end with restored airflow or a cleaned sensor; when a part has genuinely failed, we replace it and confirm a full, clean heat cycle.
DIY vs. calling a pro
Safe to try yourself first (free):
- Confirm it's a counted pattern, not the slow heartbeat (which is normal), and note the flash count.
- Turn the furnace off and change a dirty filter, wait 20–30 minutes, and restart — this clears a large share of 4-flash codes.
- Open blocked supply and return vents, and check outdoor vent terminations for blockage on a 3.
- Power-cycle at the switch or breaker to reset a soft lockout after fixing airflow.
Call a pro when:
- A code returns after a fresh filter (2, 3, 5, 7, 8, 9 especially), or you see a 6 (electrical polarity/ground).
- The furnace locks out with no heat, or you smell a hot/burning odor.
- You need the 7 or 9 meaning confirmed against your specific board.
Gas, line-voltage electrical, and any refrigerant or compressor work belong with a licensed technician (EPA 608 certified for refrigerant handling) — a furnace that keeps overheating can crack the heat exchanger, which is a carbon-monoxide risk.
What it costs in the Bay Area
Many flashing-red-light fixes are inexpensive because the cause is airflow or a dirty sensor. When a part is needed, in our pricing a flame sensor runs about $90–$250, a control board $200–$600, and a blower motor $400–$700, depending on model. We charge a diagnostic fee that's credited toward the repair when you approve the work, with a firm, upfront price before any work begins.
Book Trane furnace service
Triton services Trane and American Standard furnaces — plus every major brand — across the SF Bay Area & Peninsula, with same-week and often same-day appointments. If your Trane is flashing a code, we'll read it correctly and get your heat back.
Call (888) 993-5699 or book online for same-day furnace service.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Furnace Not Heating / No Heat — the symptom behind a lockout
- Furnace Short Cycling (On and Off) — overheating that cycles
- Trane / American Standard HVAC Repair — brand service hub
- Furnace Repair in Redwood City — same-day service in your city
Frequently asked questions
What does a flashing red light on my Trane furnace mean? It depends on the rhythm. A slow, steady heartbeat blink is normal and means the board is healthy. A counted pattern, where the same number of quick flashes repeats after a pause, is a diagnostic code identifying why the furnace shut down. Trane and American Standard furnaces use the same boards and chart.
Is a slow blinking light on a Trane furnace normal? Yes. A slow, rhythmic heartbeat on the control board LED is the normal healthy signal. It only becomes a fault code when it changes to a counted burst, such as four quick flashes, a pause, then four again. Count that repeating burst to read the code.
What does 4 flashes mean on a Trane furnace? Four flashes is an open high-temperature limit: the furnace overheated because air was not moving fast enough. The usual causes are a dirty filter, restricted ducts, or closed vents. It is the most DIY-friendly code. Start with a fresh filter and open registers, then restart.
What does 3 flashes mean on a Trane furnace? Three flashes is a vent or pressure-switch error: the inducer is not proving a proper draft. Common causes are a blocked intake or exhaust vent, a weak inducer motor, a clogged condensate line, or a pressure-switch hose fault. Check outdoor vent terminations, then call a tech if it returns.
What do 7 or 9 flashes mean on a Trane furnace? These two vary by board generation. A 7 is a gas-valve circuit fault on some boards and an ignition lockout on others; a 9 is an igniter-circuit check on some and an open limit on others. We do not guess. We read the code on your specific board and confirm it against the diagnostic label inside your unit.
How much does Trane furnace repair cost in the Bay Area? Many fixes are inexpensive because the cause is airflow or a dirty sensor. When a part is needed, a flame sensor runs about 90 to 250 dollars, a control board 200 to 600 dollars, and a blower motor 400 to 700 dollars, depending on model. We credit the diagnostic fee toward the repair and quote a firm price first.