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6 Lawn Mower Deck Problems (With Troubleshooting Tables)

Too often lawn mower owners wait for their deck to start making noise or smell like burnt rubber before they consider looking at their deck.

Not taking time to look at the mower deck before this happens can result in a very costly repair. Don’t make this mistake with your lawn mower deck.

The way your lawn looks a day or two after it is mowed can indicate it’s time to look at your lawn mower deck for repair.

These early indications can signify mower deck problems:

  • Tips of your grass appear brown a couple of days after you mow
  • The cut of your lawn is uneven
  • Uncut strips of grass
  • Mower deck falls off
  • Deck begins vibrating

6 most common mower deck problems:

  • Uneven cut
  • The belt keeps falling off or breaking
  • Deck is Vibrating
  • Blades won’t engage
  • A buildup of grass clippings and debris under the deck
  • The Mower deck is squealing

Always remove the spark plug wire and ignition key before starting any deck troubleshooting and repairs. Wait for all parts to stop moving. Wear safety gear.

mower deck packed with grass and dirt

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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Common Mower Deck Problems

1. Bad or Uneven Cut with Your Mower

The mower blades create air movement under the area of the deck to form a suction. This suction lifts grass while the blade passes by to give it a nice cut.

When you no longer achieve a good cut, first check the tire pressure. If you have a low tire, your mower deck will no longer sit level.

Once you have verified the tire pressures are right, check the condition of the mower blades and deck components for wear or damage.

Detailed information can be found at:
Reasons a Lawn Mower Has a Bad or Uneven Cut

Mower blade is dull or wornSharpen a dull blade. Replace if the sail is thinner than the center of the blade, the end of the blade is rounded, or the blade has gouges.
Mower blade is bentReplace a bent blade. Do not attempt to straighten it.
Mower blade is installed wrongInstall the blade with the correct hardware. The blade sails must point upward toward the deck.
Mower deck is plugged with grassScrape the deck to remove grass clippings.
Tire pressure is lowCorrect the tire pressure. Make sure the left and right tire pressures are equal.
Mower deck isn’t levelLevel the deck using the instruction in the operator’s manual.
Blade spindle is bent / Spindle housing bearing damage.Replace a bent spindle. Check the spindle bearing and replace is worn or damaged.
Worn deck beltReplace a stretched, worn, cracked, or shredded belt.
Bad pulley bearingReplace a bad pulley bearing restricting pulley rotation or making the pulley wobbly.
Engine speed is too lowAlways run the engine at full throttle when engaging the blades.
Ground speed is too fastSlow down and mow according to the conditions. Slow down when cutting thick, tall, or wet grass or when operating on inclines.
Incorrect overlapping path / grass is too tallProperly overlap your paths so you don’t leave strips of grass. Double or triple-cut grass when it is very tall.
Mower deck shell is damagedAttempt to straighten the deck if possible. Replace the deck shell or complete deck assembly if it is not.
Lawn is unevenLevel an uneven lawn using a lawn roller to avoid scalping. Use a small format mower for ditches.
Law Mower Has a Bad or Uneven Cut

2. Mower Deck Belt Keeps Coming Off

The deck belt keeps falling off your mower when deck components are worn, debris is stuck in the deck components, or the belt is worn or damaged.

When proper tension isn’t able to be placed on the belt, the belt can become loose and come off the pulleys.

Detailed information can be found at:
Deck Belt Keeps Coming Off or Breaking

Stretched or worn mower deck beltReplace a stretched or worn mower deck belt.
Belt keeper is damagedAttempt to bend the keeper back to its original position. Replace a damaged belt keeper.
Deck belt is shreddedReplace a shredded belt. Find and fix the item the belt is rubbing against. Make sure the belt is routed around the pulleys correctly.
Bad pulley bearingReplace a bad pulley bearing or pulley assembly restricting pulley rotation or making the pulley wobbly.
Bad spindle housing bearingReplace a bad spindle housing bearing or spindle housing assembly.
Idler tensioner bracket or spring is wornReplace a worn bracket or spring.
Debris is interfering with proper tensionRemove debris collecting around and under the tensioner. Make sure the tensioner is able to move.
Debris buildup in the pulley groovesRemove debris collecting under the pulleys or in the pulley grooves.
Oil on your deck beltFix an oil leak. Remove oil from the mower deck and components. Replace with a new belt.
Rust on your deck beltRemove rust from deck components. Replace any components that are extremely rusty. Replace the mower deck belt.
Mower Deck Belt Keeps Falling Off or Breaking

3. Mower Deck is Vibrating

You’re feeling more vibration than normal and it’s coming from your mower deck. A common reason for this is unbalanced mower blades.

Blades can become unbalanced, with one side of the blade heavier than the other, from normal wear or during the blade sharpening process.

You may also find failed bearings in the deck components, a loose belt, a worn clutch, or debris lodged in the mower deck.

Detailed information can be found at:
Reasons a Mower is Vibrating

Clutch is wornReplace an electric clutch with ball bearing failure. Check the belt, pulley, bushings, and pulleys for failure on a manual engagement clutch.
Mower blades are installed wrong Install the blade with the correct hardware. The blade sails must point upward toward the deck. Tighten the nuts to the specification listed in your operator’s manual.
Mower blades are unbalancedBalance the blade so they are of equal weight on each end of the blade. Remove a little metal for the heavier end until the blade is of equal weight.
Foreign material is wrapped around the blade shaftRemove any material that has wrapped itself around the blade shaft. Inspect the spindle housing for damage.
Bad bearing in the spindle housingReplace a bad spindle housing bearing or spindle housing assembly.
Deck belt is loose or stretched Replace a stretched, worn, cracked, or shredded belt.
Bad pulleyReplace a bad pulley bearing or pulley assembly restricting pulley rotation or making the pulley wobbly.
Debris lodged in the mower deckInspect the mower deck and remove any debris and foreign objects.
Mower is Vibrating or Shaking

4. Mower Blades Won’t Engage

Blades won’t keep rotating under the deck when the belt is worn and no longer grips the pulleys to turn the blades.

You will also need to check for items that can fail to engage the clutch solenoid on an electric clutch or a worn clutch cable, linkages, and bushings on a manual clutch.

Detailed information can be found at:
Mower Blades Won’t Engage or Turn On

Mower deck belt is wornReplace a stretched, worn, cracked, or shredded belt.
Belt has come off the pulleysLook for a worn tensioner arm or spring; loose belt, or worn pulley bearings.
PTO switch is faultyReplace the PTO switch.
Clutch is BadReplace a bad electric clutch.
Clutch cable or linkages are worn on a manual clutch modelReplace a worn clutch cable, bushings, or linkages on a manual clutch.
Battery is weakCharge a weak battery. Replace a bad battery that is no longer able to hold a charge.
Safety switch is badReplace a bad safety switch.
Fuse is blownReplace a blown fuse with the same capacity fuse. Have a mechanic find the root cause if the mower continues to blow a fuse.
Mower Blades Won’t Engage or Rotate

5. Mower Deck Builds Up with Grass Clippings

When the mower deck builds up with grass, it will no longer have a sufficient area for good airflow. It will also cause the engine to work harder to turn blades through a deck packed with grass.

To minimize grass buildup, you can try to use a deck spray. While this spray won’t keep all grass from sticking, it will reduce the amount of grass that sticks.

It’s important to keep the area under the deck free of debris and grass clippings by routinely scraping the mower deck.

Mowing wet grassOnly mow dry grass.
Mowing at a low engine speedMow at a high throttle to create better air movement under the deck.
Mowing long grassMow more frequently to avoid cutting long grass. If the grass is long, you should double or triple-cut the lawn.
Reasons Grass Keeps Building Up Under a Zero Turn Mower Deck

6. Mower Deck is Squealing

When you hear an odd noise or a squealing coming from the mower deck, the first thing to look at is the belt. If the belt has slipped and is stuck on the deck, reinstall the belt correctly around the pulleys.

Next check the pulley and spindle housing bearings. This noise could be an indication of a bearing failure.

Belt slipped and stuck between the belt guides.Reinstall the belt correctly. Replace the belt if it is worn.
Broke tensioner spring Replace the tensioner spring.
Material wrapped around the blade shaftRemove any foreign objects wrapped around the blade shaft.
Bad spindle housing bearingReplace the spindle housing assembly. You may able to replace the bearing on some zero-turns.
Bad pulley bearingReplace the pulley assembly. You may be able to replace the bearings in some zero-turn pulleys.
Zero Turn Mower Deck is Squealing & Making Noise

7 Tips for Finding a Mower Deck Problems

It is always best to start with the easy items and work your way to the more difficult deck problems. These problems can exist in all versions of the mower deck including those on walk-behind, zero-turn, and riding mowers.

Remove Spark Plug Boot and Key From Your Lawn Mower for Safety.

Safety is key when working on your mower deck. It is important to remove the spark plug boot and the ignition key from your lawn mower.

I like to stick the key in my pocket so no one picks it up and tries to start the mower not knowing I’m working on it.

Wear heavy work gloves when working with your mower blades as they can be sharp. Follow additional safety tips as shown in your operator’s manual.

Check the Condition of Your Mower Blades

As mentioned earlier, the tops of your grass can become brown a couple of days after you mow. This happens because worn mower blades will beat the grass instead of precisely cutting it.

The ends of the grass are beaten up and will slowly turn brown. Dull blades must be sharpened or replaced. I go into more detail on sharpening, replacing, and balancing blades in “Inspecting Your Lawn Mower Blades Saves You Time & Money“.

Bent mower blades can also affect the cut of your lawn. Sometimes a bent blade is very noticeable and other times it is hard to tell. You can easily check for a bent blade by parking your mower on a flat surface.

You will need to measure one end of the mower blade from the ground to the blade tip. Then rotate the blade and measure the opposite end of the blade from the ground to the blade tip. If there is over a 1/8″ variance, replace it with a new blade.

Repeat this step with the other mower blades if your mower deck utilizes more than one blade. Never attempt to straighten a mower blade as a straightened blade will be weakened and compromised. You don’t want a weak blade rotating at high speeds.

Check Your Mower for Low Tire Pressures

Did you know something as simple as the wrong air pressure in the tires can cause you to end up with a bad mower cut? Check your tire pressure. When you have a low tire, the mower deck no longer sits parallel to the ground which causes uneven cutting.

Make Sure Your Deck Height is Level

You need your mower deck to sit level for a good cut. The check this, you need to measure the deck at all four corners from the ground to the bottom of the deck.

If you have a variance in your deck, refer to your owner’s manual for proper instructions to level the deck. This step varies with the manufacturer and model of the mower.

Check the Bearing in Spindle Housing

Next, move you should check the bearings in the spindle housings. With a good pair of work gloves, grab a hold of each end of the mower blade and rock it up and down.

If you feel any movement or noise, you may have a bad bearing in the spindle housing that must be replaced.

The bearing can be replaced in some manufacturers’ spindle housings. Other manufacturers will only allow you to buy the whole assembly.

Check the Condition of Your Mower Deck Belts

Your mower belt must be in good condition without any cracking, wearing, or glazed appearance to it. Replace the belt if needed.

Check Mower Deck Pulleys & Idler Pulley Brackets

Your next step is to check your mower deck pulleys and idler brackets. With the deck belt removed from your mower, spin the pulleys to make sure they are moving smoothly and not making noise.

If you do hear a noise or there is any restriction in movement, you need to replace the pulley. Check the idler pulley bracket to ensure there isn’t any play where it mounts to the deck.

The idler bracket must be kept greased so it doesn’t seize up. To prevent the belt from breaking, the idler pulley bracket is designed to move and spring back to put tension back on the belt.

If the bracket is unable to move, the deck belt can slip and cause the blades to also slip in heavy or deep grass so our cut looks bad the next day.

A Clogged Mower Deck Can Affect Quality of Cut

Grass can affect the quality of the cut. Keeping your mower deck clean of grass buildup is essential. Besides scraping your deck during regular service maintenance, there are other steps you can take to avoid grass buildup.

  • Avoid cutting wet grass.
  • Always run your mower at full throttle when the mower deck is engaged.
  • Cut your grass regularly so you don’t have clumping of grass.
  • Coating your cutting deck with silicone deck spray can help minimize the amount of grass collecting under your mower deck.

It is necessary for the performance of your mower to keep your mower and mower deck clean.

How Often Should You Perform Maintenance on Your Lawn Mower Deck?

The mower deck is the main component of your mower, but I think it is often the most neglected.

For the average homeowner, mower deck maintenance should be performed at least twice per mowing season. If your yard is heavily watered or fertilized you need to perform maintenance more often.

The grass is full of moisture and fertilizers, if you use them, that can create erosion of the deck so you may not get the life you expected out of your lawn mower deck. Scrape and clean out the undersides of your decks more than once a year.

While you are under the deck, use the steps above to identify all mower deck parts are functioning as needed and are in good condition. Performing these routine checks will help you prevent more costly repairs.

Lawn Mower Decks Are Not the Same

Mower decks look pretty similar. Is there really a difference? There are significant differences to consider in a lawn mower deck when making a purchase. These are important items that will affect the quality and durability of your mower deck:

  • Whether the lawn mower deck is a stamped deck or fabricated deck
  • The thickness of the steel used for the mower deck shell for durability
  • Lawn mower deck depth affects the quality of cut
  • Quality and durability of lawn mower parts
  • The use of greaseable parts for easy maintenance results in fewer failures
  • How easy is it to find and buy parts for the mower deck to keep your lawn mower deck in good condition?

What is Causing My Mower Deck to Vibrate?

Mower deck vibration can be caused by bad bearings in the cutter housing, unbalanced blades, or debris wrapped around the blade shaft or lodged in the components of the mower deck.

Other parts of your mower may also be causing the mower to vibrate. Check out our article “How to Stop Your Lawn Mower from Shaking and Vibrating”.

Still Having Problems with Your Lawn Mower?

Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.

For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.