Trane furnace E12 is a full ignition lockout after three failed ignition attempts. The furnace went through the full startup sequence (inducer on, ignitor glowing, gas valve opening) at least three times without detecting a confirmed flame, then locked out. Start by cleaning the flame sensor rod with 0000 steel wool, as sensor fouling is the most common cause.
Here's what I see nine times out of ten on an E12 call: a flame sensor that hasn't been touched in years. You've got maybe a $15 fix staring you in the face. But ignore this and your furnace just won't run through the night, which in January means frozen pipes territory. Clean the sensor first, then work through the ignitor and gas pressure if it's still acting up.
Multimeter with microamp DC and resistance modes, 1/4-inch nut driver
What Does the E12 Code Mean?
Trane units are picky about flame rectification signals, honestly pickier than most other brands I work on. Even a slightly dirty sensor or a weak ignitor that isn't pulling enough current will trip this lockout every time. Before you assume the gas valve's shot, you need to verify the ignition sequence is actually completing and that the control board is getting the microamp signal it expects. I fixed three of these last week alone and two of them were just a dirty sensor.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Flame sensor rod coated with oxide film blocking microamp signal40%
Hot surface ignitor cracked or failed to reach ignition temperature24%
Gas valve not opening at correct time or not fully opening22%
Gas supply pressure low or manual shutoff valve partially closed14%
Symptoms You May Notice
E12 displayed on the furnace control board digital display
Furnace completely unresponsive after multiple startup attempts
Ignitor visibly glows during startup but no sustained flame
Furnace ran for a few cycles earlier then stopped lighting
No clicking from gas valve or gas valve clicks but no ignition
Can you reset a Trane furnace to clear the E12 code?
Find the power switch on the side of the furnace, looks like a light switch, and flip it off. Wait 30 seconds, then flip it back on. You can also hit the breaker if you can't find the switch. Some folks toggle the thermostat from Heat to Off, wait 30 seconds, then back to Heat. But resetting without fixing the underlying problem just gives you three more ignition attempts before it locks out again, so don't keep hammering the reset without diagnosing first.
Tools Required for Diagnosis
Multimeter with microamp DC and resistance modes1/4-inch nut driver0000 steel wool (fine grade only)Phillips #2 screwdriverFlathead screwdriverWork glovesFlashlight or headlamp
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range40–90 ohms
ConditionIf Open (OL) or infinite, replace component.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Hot Surface IgnitorSEN00107 · $25–$50
SEN00107
$25 – $50
Flame SensorSEN00885 · $10–$20
SEN00885
$10 – $20
Frequently Asked Questions
How many times does a Trane furnace try to ignite before showing E12?
Three tries. The board runs a full sequence each time: starts the inducer, waits for the pressure switch to close, glows the ignitor for about 30-45 seconds, opens the gas valve, then gives the flame sensor roughly 4 to 10 seconds to report back a signal. If it doesn't get one, it shuts down, waits a couple minutes, and tries again. After the third failed attempt it triggers E12 and stops entirely. You've got to manually reset the power to get another three shots at it.
Why does my Trane furnace sometimes light and other times show E12?
That's basically textbook dirty sensor behavior. The carbon or oxide coating on the rod is semi-conductive, meaning sometimes just enough signal squeezes through to satisfy the board and sometimes it doesn't. Humidity in the basement makes it worse. I've also seen this a lot with a hairline-cracked ignitor. It works fine when cold, but as the element heats up and expands during the glow phase, the crack opens up and kills the circuit right before the gas catches. If it's intermittent, start with the sensor but don't rule out the ignitor.
How much does it cost to fix a Trane E12 error?
If it's the flame sensor, you're looking at $10-20 for the part and maybe 20 minutes of your time. Ignitor replacement runs $25-60 for the part depending on the model. If you call a tech out, figure $150-300 for a service call plus parts. Gas valve replacement is the expensive fix, usually $200-400 for the part alone plus labor. Honestly, try the flame sensor clean first. That solves it probably 60-70% of the time and costs you nothing but a piece of steel wool.
Can I just keep resetting the E12 to run my furnace temporarily?
You can, but don't do it more than a couple times without figuring out why it's failing. Each lockout means the board ran through a failed ignition sequence where gas was briefly released without being burned. And if it's the middle of winter and you're resetting it every few hours to keep the heat on, you're probably one cold night away from it failing completely. Fix the sensor or ignitor now before you're dealing with frozen pipes at 2am and every HVAC tech in your area has a 3-day wait.
What's the difference between E12 and other ignition-related codes on Trane?
E12 is specifically the full lockout after three failed attempts where the furnace actually got to the ignition stage and tried to light. Some Trane boards also throw codes for pressure switch faults, rollout switch trips, or limit switch faults that can look similar if the furnace just won't start. The key thing with E12 is that it got past the inducer and pressure switch stages and made it to the ignitor glow and gas valve open phase. It just couldn't confirm a flame. If it's not even getting to the ignitor glow, you're probably looking at a pressure switch or inducer issue instead, which is a totally different diagnostic path.