On the covered York YP9C modulating gas furnace, 4 red flashes means high limit switch open, or a defective temperature sensor, or the 24 volt fuse is open. First, check that the air filter is clean and the supply and return registers are open.
Differential
Likely causes of “York YP9C modulating gas furnace showing 4 red flashes”, and how to tell them apart
- 1
A dirty air filter
What separates it Weak airflow and a furnace that shuts off after a few minutes
How to confirm Replace or clean the filter
- 2
Closed or blocked registers
What separates it Rooms with no airflow
How to confirm Open the supply and return registers
- 3
A blower motor problem
What separates it The blower is slow or noisy
How to confirm Technician testing
- 4
An open 24 volt fuse on the control board
What separates it Nothing responds and the code shows
How to confirm Technician replacement
- 5
A defective temperature sensor
What separates it The limit trips with good airflow
How to confirm Technician testing
Before you touch a setting
Record this first — changing settings destroys the evidence
- That the LED shows four red flashes
- When the filter was last changed
- Whether supply and return registers are open
- Whether the blower runs
Try in this order
0 of 8 completedWhat to do first
ReversibleCheck that the air filter is clean and the supply and return registers are open.
Correct only the documented fault domain
ReversibleYork's manual says this may be caused by a dirty air filter, an improperly sized duct system, a faulty blower motor, restricted circulating airflow, or an open fuse on the control board. Filter and registers are the safe checks; the rest is technician work.
Turn the thermostat down
ReversibleAnd wait for the furnace to cool.
Replace or clean the filter
ReversibleCheck its direction arrows.
Open every register
ReversibleAnd clear returns.
Restart the furnace
ReversibleWatch the blower.
Time how long it runs
ReversibleBefore the code returns.
Call a technician
ReversibleIf it returns with a clean filter.
Model-specific
What is true of this model and not of its siblings
- The limit switch
- The high limit switch opens when the furnace's heat exchanger gets too hot, and it shuts off the gas to stop damage. York lists a dirty filter, an ill-sized duct system, a faulty blower motor, restricted circulating airflow or an open fuse as causes.
- Filter first
- A clogged filter is the simplest cause and the easiest fix. Look for a filter slot at the furnace or a return grille, take the filter out and hold it to the light. If you cannot see through it, replace it.
- Registers and returns
- Closing many registers raises the pressure in the ducts and cuts the airflow. Keep at least most registers open, and check that furniture or rugs do not block returns.
- A pattern
- If the furnace runs a few minutes, then shuts off and restarts later, the limit is probably cycling. A longer run before the trip points to a milder restriction.
- What a technician does
- Measures the temperature rise across the furnace, checks blower speed and ducts, and tests the sensor and fuse.
- Do not bypass
- Never jumper a limit switch. It is the last guard against overheating.
- How to change a filter well
- Turn the furnace off, slide out the old filter, and check the size printed on its frame before you buy a new one. Fit the new filter with the arrow pointing toward the furnace, in the direction of the air flow. Write the date on the frame. In a home with pets or a lot of dust, a filter may need changing every month or two, while in a clean home three months is common.
- Duct problems that look like limit trips
- York mentions an improperly sized duct system. A new duct that is too small, a crushed flexible duct in the attic or a closed damper can restrict the air. If the furnace has worked for years and the trips began after a renovation, say so to the technician, who will measure the temperature rise.
- A pattern to watch
- A furnace that trips on the limit shortly after starting often has a blower or airflow fault, while one that trips after running a long time may be overheating at the end of a long cycle. Note how long the furnace runs before each trip, because the timing helps point to the cause.
Stop boundary
When this stops being a self-service repair
- !Never bypass, jumper or tape over a safety switch
- !Stop for a carbon monoxide alarm, soot, scorch marks or flame outside the burner compartment
- !Call a qualified technician for anything behind the furnace's panels
- !If you smell gas, leave the building and call the gas utility from outside; do not use switches or phones inside
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 4 red flashes mean on a York YP9C modulating gas furnace?+
High limit switch open, or a defective temperature sensor, or the 24 volt fuse is open.
What causes it most often?+
Restricted airflow, often a dirty filter.
Can I fix it myself?+
Changing the filter and opening registers is safe.
Why does it return?+
The airflow problem is still there.