Trane furnace E08: Flame current low but still usable
E08 means the flame current has fallen but is still strong enough for the furnace to keep running. It is a warning, and it is the cheapest moment to fix the problem.
Applies to: Trane and American Standard S9V2 two-stage condensing variable speed gas furnaces with a seven-segment integrated furnace control display, per Installer's Guide 18-CE01D1-1E-EN.
What it means
This is the code that arrives before the furnace starts failing. The flame signal is weakening but has not yet dropped below the threshold that forces a shutdown, so the furnace keeps working and nobody notices. Left alone it becomes E2.2, recycles exceeded, which is a furnace that lights and shuts down repeatedly. Trane publishes the numbers needed to act on it, which is what makes this control unusually good to work on. There are two flame sense pads on the integrated furnace control marked FP. A meter set to DC volts reads them directly, and one volt DC equals one microamp of flame current. Typical flame current runs 0.75 to 3.0 microamps, which is 0.75 to 3 volts DC on that meter. That is a specific, checkable range rather than a judgement.
Most common causes
- A flame sensor coated in oxide, which builds up normally over a heating season.
- A poor ground or reversed line polarity, both of which Trane reports separately as E6.2 and E6.1.
- A sensor cracked or positioned so the flame does not fully envelop it.
- Burners partially blocked so the flame does not reach the rod properly.
- Gas pressure low enough to give a small or lifting flame.
How to diagnose it
- Treat this as a maintenance prompt rather than a fault. Acting now avoids a no-heat call later.
- Ask for the flame current to be read at the FP pads with a meter on DC volts, and compare against the 0.75 to 3.0 microamp range.
- Check the Last 6 Faults menu for E6.1 or E6.2, which would explain a low reading with a clean sensor.
- Note when the flame sensor was last cleaned.
How to fix it
- Cleaning the flame sensor is the usual repair and is technician work in the burner compartment.
- Ask for the reading before and after, since this control makes the improvement measurable.
- If cleaning does not raise the reading, the cause is ground, polarity, or flame quality — not the sensor.
Notes
The published measurement method is the most useful thing on this page. Most manufacturers give a threshold without telling you where to measure it; Trane gives the pads, the meter setting and the conversion.
Related
All codes for this manufacturer
Trane and American Standard furnace flash codes — every code, with what differs between models.