The quick answer
The code is a measurement, not a guess. The dryer compares the air it pushes out against what a free duct would take, and the gap is the number on the panel. Nine times in ten the gap is the vent run: a crushed flex behind the dryer, a run with too many bends, lint packed into the duct or the hood outside. The sensor itself is one of the last things to fail, and swapping it without fixing the run brings the code straight back.
The two-step test
Ten minutes, no tools beyond a screwdriver for the clamp. Each step rules out one half of the problem.
- 1
Run it with the hose off
Pull the dryer out, take the vent hose off the back and run a load venting into the room (hang a towel over the opening to catch lint). If the code stays away and the load dries fast, the sensor and the dryer are fine. Everything from here on is the vent.
- 2
Run it on a short piece with one bend
Put a two- or three-foot piece of metal duct on the back with a single 90-degree elbow and run it again. If it still runs clean, the dryer handles a normal connection and the problem is the run in the wall, the ceiling or the hood outside.
- 3
Connect the real run and watch the flap
Hook the full run up and start the dryer. Go outside: the hood's flap should swing open and stay open while it runs. If it barely moves, the run is restricted somewhere between the dryer and the hood. If the code comes back here and nowhere else, the run is the whole story.
A dryer that throws the code with the hose off, in a room with the door open, has a problem inside: a lint-packed blower housing, a duct inside the cabinet that came loose, or the sensor itself. That is a service call, and on a machine we sold it is a call to us first.
The airflow codes
Every maker names the same measurement differently. These are the ones that mean “restricted exhaust”; the fix is the same for all of them.
| Maker | Code | What it reads |
|---|---|---|
| Samsung | C8, C80, C81, C90, Cg, Clg | The exhaust is about 80% (C8 / C80) or 90% (C90) restricted. Cg and Clg say the same on older panels. |
| LG | d80, d90, d95 | Flow Sense reads the duct 80%, 90% or 95% blocked. d80 is a warning; d90 and d95 stop the cycle. |
| Whirlpool, Maytag, Amana | AF, or the Check Vent light | Restricted airflow. The light stays on until the run clears; the dryer keeps running at a lower heat. |
| GE, Hotpoint | Check Vent indicator | The dryer measured low airflow on a Sensor Dry cycle. It dries, slowly, until the run is cleared. |
A code that shows only on heavy loads, towels or bedding means the run sits right at its limit. It is the same fix, sooner rather than later.
The venting rules
Short, straight, metal, and open at the end. Everything below is one of those four.
4-inch metal, nothing else
Rigid galvanized or aluminum duct for the run, joints taped with foil tape. A short piece of semi-rigid metal is fine for the connection behind the dryer. Never white vinyl flex, never thin foil flex for the run.
Count the bends as length
Building code allows 35 feet of developed length. Every 90-degree elbow counts as 5 feet, every 45-degree elbow as 2.5 feet. Keep the whole run under 25 feet with the bends counted and the dryer will never notice it.
No screws into the duct
A screw tip inside the duct catches lint, and the lint catches more lint. Tape the joints, or use clamps on the flexible piece.
Nothing crushed, nothing kinked
The flexible piece behind the dryer is where runs die: the dryer gets pushed back and the duct folds flat. Leave it a gentle curve, or use a close-elbow or periscope fitting for a tight wall.
A louvered hood, no screen
The outside hood needs a flap that swings open when the dryer runs and closes when it stops. A screen or a grille over it is a lint trap. Clear anything stacked in front of it.
Vent outside, never inside
Not into a crawl space, an attic, a garage or a chimney. The moisture goes into the house and the lint goes somewhere you cannot clean.
How the length adds up
A 15-foot run with two 90-degree elbows is 25 feet of developed length (15 + 5 + 5). A 20-foot run with three 90s is 35 feet and sits at the code limit. A 30-foot run with two 90s is 40 feet and over it, whatever duct it is in. Your dryer’s own manual may allow more with rigid metal; the code figure is the safe one.
Installing a new vent
- Lay the run out before you buy a foot of duct. Count the elbows, add 5 feet for each 90 and 2.5 for each 45, and keep the total under 25 feet. If the straight line to the outside wall is shorter than the route through the ceiling, take the straight line.
- Use rigid metal for everything in the wall or ceiling and slope any horizontal stretch slightly toward the outside so water never pools.
- Make the piece behind the dryer the last thing you fit. Semi-rigid metal, as short as it can be, a gentle curve, and a periscope or close-elbow fitting when the wall is closer than a foot.
- Tape every joint with foil tape. No screws through the duct, no duct tape (it dries out and lets go in the heat).
- Fit a louvered hood outside with no screen, and check that the flap swings freely before the dryer ever runs.
- Run the two-step test before you push the dryer back and before you call it done. A run that passes on day one stays passing; a run that throws the code on day one will not fix itself.
Connecting to a vent that was already there
A run that served the old dryer for years carries years of lint, and a new dryer with a sensor is the first thing to say so. Treat the old run as suspect until it is clean and measured.
- Clean the whole run with a rotary brush kit from both ends, then run the dryer on air-only with the hood open to blow the loose lint out. A shop vacuum at the hood catches the rest.
- Clean the hood outside: lint, paint, a stuck flap, a bird or wasp nest behind it. Take the hood off if it does not swing freely by hand.
- Look for the piece that was crushed behind the old dryer, any foil or vinyl flex, and any joint held by screws. Replace those pieces with rigid or semi-rigid metal.
- Measure the run and count the bends. A run that was already past 35 feet of developed length was slow for the old dryer too; a new dryer only says it out loud.
- Wash the lint screen with warm water and dish soap. Softener sheets leave a film that water beads on, and that film restricts airflow before the vent does.
- Then run the two-step test. Hose off, short piece with one bend, then the real run with the flap watched from outside.
What the dryer is actually measuring
Samsung, LG and Whirlpool put a small pressure or flow sensor in the blower housing, right behind the lint screen; GE reads the same thing off its moisture and temperature sensors during a Sensor Dry cycle. When the exhaust is free the pressure stays low; when the run restricts, the pressure climbs and the dryer reads the climb as a percentage. It is a simple part and it rarely fails. What does fail is the run it is reading, and a lint-packed blower housing right in front of it, which cleaning the vent does not reach. If the two-step test says the dryer itself is restricted with the hose off, that housing is the next thing a technician opens.
Every dryer we sell ships with the full manufacturer warranty, and a dryer that passes the two-step test has nothing wrong with it. If yours throws the code with the hose off, text us at (573) 353-8511 with the model and what you saw, and we will point you at the right next step.
FAQ
My dryer shows C80 but it still dries. Is the sensor bad?
Almost never. The code is the dryer measuring restriction at its exhaust, and it measures correctly far more often than it fails. Run one load with the vent hose off the back. If the code stays away, the sensor is fine and your vent run is the cause.
Can I use the white or foil flex duct from the hardware store?
Not for the run. White vinyl flex is a fire hazard and most makers forbid it; thin foil flex sags, kinks and traps lint. Use 4-inch rigid metal for the run and a short piece of semi-rigid metal only for the connection behind the dryer.
How many bends can a dryer vent have?
Count them as length. Building code allows 35 feet of developed length, with each 90-degree elbow counted as 5 feet and each 45-degree elbow as 2.5 feet. A 20-foot run with three 90s is a 35-foot run and sits right at the limit; most dryers are happiest under 25 feet with two bends or fewer.
Does a booster fan fix a long run?
A listed dryer booster fan is the right answer for a run past the limit that cannot be shortened, and only then. It never fixes a lint-packed run or foil flex; clean and rebuild the run first.
Why does the code only show on big loads or towels?
A heavy load puts more moisture and more lint into the air at once, so a vent that is almost restricted tips over the line. Your run is too close to its limit; the dryer is telling you before the dry times get long.
The flap outside does not open when the dryer runs. What does that mean?
Not enough air is reaching it. The run is blocked, crushed or too long, or the flap itself is stuck with lint or paint. Clean the hood first, then test the run with the hose off the dryer.
How often should the vent be cleaned?
Once a year for a short run, twice a year for a long one or a house with pets. Clean the lint screen every load, and wash it with soap when water beads on it.
Need a dryer that fits your hookup?
Tell us your outlet, gas or electric, and where the vent goes. We will match you to a dryer on the floor at 15 to 70% under retail. Call or text (573) 353-8511.
Related reading: Dryer Troubleshooting · Vented vs. Ventless Dryers
