On the covered York TM9V two-stage variable-speed gas furnace, 8 red flashes means the flame is lost five times (four recycles) during the heating cycle. The furnace locks out for one hour and then restarts. First, note which stage the furnace was in, low or high fire, when the flame dropped out.
Differential
Likely causes of “York TM9V two-stage variable-speed gas furnace showing 8 red flashes”, and how to tell them apart
- 1
A flame sensor coated with residue
What separates it The flame lights, reads poorly and goes out
How to confirm A technician cleans the rod
- 2
Gas pressure that falls on a high demand
What separates it Burners weaken as the furnace moves to second stage
How to confirm A technician measures pressure at both stages
- 3
A gas valve that closes early
What separates it The flame drops out at a regular point in the cycle
How to confirm A technician tests the valve
- 4
A poor ground
What separates it The sensor signal is unreliable
How to confirm A technician checks the furnace's ground
- 5
A control that cannot hold the flame signal
What separates it Parts test well
How to confirm A technician replaces the control
Before you touch a setting
Record this first — changing settings destroys the evidence
- That the LED shows eight red flashes
- Whether the furnace was in low or high stage when flame was lost
- Whether the lockout repeats each hour
- How long the furnace ran before the first loss
Try in this order
0 of 8 completedWhat to do first
ReversibleNote which stage the furnace was in, low or high fire, when the flame dropped out.
Correct only the documented fault domain
ReversibleOn the two-stage TM9V York says to check for low gas pressure, a dirty or faulty flame sensor and a faulty gas valve, in that order.
Count the flashes
ReversibleEight red, then a pause.
Note the stage
ReversibleLow or high fire, if you can tell.
Wait for the restart
ReversibleYork says one hour.
Watch the next attempt
ReversibleFrom the front, at a distance.
Check outside
ReversibleFor a blocked vent or intake.
Call a technician
ReversibleWith the times of each lockout.
Model-specific
What is true of this model and not of its siblings
- Two stages, two gas pressures
- A two-stage furnace such as the TM9V burns less gas in low stage and more in high. The gas valve must give the right manifold pressure in each. A supply that is only just adequate can hold low fire and sag when the furnace steps up, which is a typical way for flame to be lost mid-cycle.
- What York lists
- York lists low gas pressure, a dirty or faulty flame sensor and a faulty gas valve. They are checked in that order by many technicians, because the pressure test is quick and the sensor cleaning is cheap.
- Grounding
- The flame sensor works by passing a tiny current through the flame to the burner and back to the control through the furnace's ground. A rusty or loose ground connection weakens the signal, so the technician checks it.
- When to call the utility
- If a gas range or water heater in the same home also acts weakly, the supply may be the issue. Call the gas utility, who can test the meter and the line.
- Counting the cycles
- Because the lockout lasts an hour, a furnace may seem to heat in bursts. Note the times of each lockout and give them to the technician, since a timed pattern is a clue.
- Prevention
- A yearly check of the burners, the sensor and the pressure catches most of these faults in autumn instead of midwinter.
Stop boundary
When this stops being a self-service repair
- !If you smell gas, leave the building and call the gas utility from outside
- !Never bypass, jumper or tape over a safety switch
- !Call a qualified technician for anything behind the furnace's panels
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 8 red flashes mean on a York TM9V two-stage variable-speed gas furnace?+
The flame is lost five times (four recycles) during the heating cycle. The furnace locks out for one hour and then restarts.
What does the furnace do next?+
It locks out for an hour and tries again.
Does two-stage matter?+
Gas pressure needs differ between stages.
Can I clean the sensor?+
A technician should.