A fault code is the most useful thing a Trane tells you and the most commonly misread — partly because Trane uses two completely different reporting systems depending on the equipment generation. This page is the Trane-specific companion to our general furnace repair service and our Trane brand overview, and it splits the two contexts so you are not misled.
Two systems, two places to read the code
Be clear which one you have before you read anything:
- Conventional Trane (older furnaces, air handlers, single-/two-stage units): a diagnostic LED on the control board behind the lower access panel blinks the code. You read it through the inspection port and count the flashes.
- Communicating Trane (XV and XL systems with an XL850, XL950, or XL1050 ComfortLink II thermostat): there is no blink code to count. The fault appears as an alphanumeric Err ###.## code on the thermostat, and you retrieve the detail in the Service menu under Diagnostics, where alerts are sorted by severity (normal, major, critical).
The two systems do not use the same numbering, so a “code 3” blink on a conventional board and an “Err 33” on a ComfortLink thermostat are read in different places and mean different things. Below, each section is labeled by context.
Conventional control-board LED blink codes
Per Trane’s own guidance, the baseline patterns are:
- Slow green blink — normal, no call for heat (standby).
- Fast green blink — normal, a call for heat is active.
- Repeating red blink pattern — a fault. Count the flashes; each sequence points to a different subsystem.
The most commonly seen numbered fault patterns across Trane and American Standard control boards:
- 3 flashes — pressure-switch fault (the switch did not close, or it stuck). Often a clogged condensate trap, a blocked or sagging vent, or a weak inducer.
- 4 flashes — open high-limit switch, i.e. the furnace overheated. Usually restricted airflow: a clogged filter, blocked returns, or a dirty blower wheel.
- 5 flashes — flame sensed when the gas valve is off (a flame or gas-valve fault).
- 6 flashes — line-voltage polarity reversed or a poor ground.
Important honesty note: the exact number-to-fault map differs by control-board generation, and some boards use the count differently. We treat the blink count as a starting point and confirm the fault on the meter — we do not condemn a part on the flash count alone. The airflow-and-limit chain behind the 3- and 4-flash faults is walked through in our furnace short-cycling guide.
ComfortLink II alphanumeric Err codes
On a communicating system the thermostat shows an “Err ###.##” code. These are the ones we field most, with their Trane meanings:
| Err code | Trane meaning |
|---|---|
| Err 22 | Ignition lockout — recycle (flame proven then lost ~10×) or retry (3 failed ignition attempts) |
| Err 26 | Limit circuit open (aux, high, or inducer limit) — overheating / low airflow |
| Err 33 | Line-voltage polarity reversed |
| Err 34 | Flame error — flame sensed >5 seconds when none should be present |
| Err 79 | Low-pressure cut-out lockout (LPCO) — low charge or airflow restriction |
| Err 80 | High-pressure cut-out lockout (HPCO) — overcharge, dirty coil, or fan failure |
| Err 87 | Flame rollout open — air impingement, bad inducer, or restricted heat exchanger |
| Err 91 | Communication error on the four-wire bus (blower, inducer, or system comm) |
| Err 126 | Equipment change alert — a discovered communicating component is no longer reporting |
These map to Trane’s published ComfortLink II alert list. A few, like Err 79 and Err 80, are refrigerant-circuit lockouts that need manifold gauges to diagnose; others, like Err 22 and Err 34, are ignition and flame faults we confirm with a flame-sense microamp reading. The ignition chain is covered on our Trane furnace not heating page and in our furnace ignitor failure and furnace blowing cold air guides.
Reading the code is the start, not the fix
Here is the honest part. A code — blink or alphanumeric — names a subsystem, not a failed part. An Err 22 can be a $185–$295 flame-sensor clean or a gas-valve problem; a 4-flash limit fault can be a filter you change yourself or a heat-exchanger problem you should not ignore; an Err 79 can be a minor charge issue or a real leak. We read the code on arrival, then confirm it on the meter — flame-sense microamps, draft pressure at the inducer, limit continuity, bus voltage, refrigerant pressures — and only then name the part. We do not guess from the code alone, and we do not jumper a safety to force a system to run. The flat $89 diagnostic ($149 after-hours) credits to the repair, so the read-and-confirm step costs you nothing if you proceed.
The SoCal first-cold-night pattern
Most furnace codes surface on the same night. An LA-basin furnace runs 200–400 hours a year against 1,500-plus in a cold climate, sitting idle from April through October. Over that off-season the flame sensor collects dust (Err 22 / 5-flash), the condensate trap clogs (3-flash pressure-switch fault), the filter quietly loads up and the limit trips on the first hard fire (4-flash / Err 26), and the ignitor cracks. The first November cold snap stress-tests all of it at once, which is why the first cold week is our busiest furnace stretch. Mountain installs in Big Bear and Wrightwood run real heating hours and fail on a wear curve instead. Fall maintenance in October is the cheapest insurance.
Trane diagnostic pricing
Reading and confirming the code is the $89 diagnostic ($149 after-hours), credited to the repair. The repair price follows the cause, flat-rate from our SoCal tickets: flame sensor $185–$295, hot-surface ignitor $245–$485, gas valve $385–$685, integrated control board $480–$950, draft inducer $580–$1,100. A cracked heat exchanger ($1,500–$3,500) is the one fault where we quote replacement against repair and red-tag the furnace if it is leaking. Full table on our Trane furnace repair page; refrigerant-circuit faults (Err 79/80) are priced with the cooling side on Trane AC not cooling.
American Standard and Trane
American Standard and Trane share the control platform — conventional American Standard boards blink the same patterns, and American Standard communicating systems display the same Err ###.## codes through the equivalent AccuLink control. We diagnose them identically. For the cooling side of a Trane system, see Trane AC repair, and the full lineup on our Trane brand page.