Fall in Longmont is hard to beat. The cottonwoods along the St. Vrain turn gold, the older neighborhoods off Main St light up with color, and for a few weeks every driveway and parking lot is covered in leaves.

Here's the part most drivers never see: a lot of those leaves don't stay on the ground. They end up under your hood.

Our ASE-certified techs, Tyler, Joe, and Alex, pop a lot of hoods this time of year, and it's common to find leaves packed in at the base of the windshield, tucked around the air intake, and sometimes turned into a cozy nest by a mouse who moved in when the nights got cold.

At Christian Brothers Automotive Longmont, local owners Josh and Lindsy Jeanneret built our shop around "The Nice Difference," and that means helping you understand your vehicle before you spend a dollar on it. So here's what fall leaves can do under your hood, and what to watch for.

1. The Cowl: Where the Leaves Pile Up

Open your hood and look at the area right below the windshield, where your wipers sit. That's the cowl. It's usually covered by a plastic panel with slots or vents in it, and underneath, it's shaped like a shallow tray.

The cowl does two jobs. It's where your heater and A/C pull in fresh outside air, and it's built to catch rain and snowmelt running off your windshield and send it out through small drain openings.

Leaves fall right onto your windshield, slide down, and collect in that tray. If you park under a tree for a few weeks in October, it can fill up fast.

2. Clogged Cowl Drains: How Water Gets Inside Your Car

When leaves clog the cowl drains, water has nowhere to go. It sits in the tray, and on many vehicles it can overflow into the fresh-air intake or find a way into the cabin.

Signs that water is getting in:

• A damp carpet or a wet passenger-side floor
• Windows that fog up on the inside and are slow to clear
• A musty or mildew smell when the fan comes on
• Water stains or dampness under the dash

A wet carpet isn't just unpleasant. It can lead to mold, and on some vehicles, electrical modules and wiring sit low enough under the dash or seats to be affected by water.

3. Leaves in the Fresh-Air Intake and Cabin Air Filter

Because the cowl is where your HVAC system breathes, leaves and debris can get pulled in toward the cabin air filter. A filter packed with leaves and dirt chokes the airflow for your heat, A/C, and defrost, and on a frosty morning, a defroster that can't keep up is a visibility problem.

We covered this in our heater article a few weeks ago. Your cabin air filter is checked during the free courtesy inspection that comes with any service.

4. Critters Moving In for the Winter

When the nights start dropping below freezing, mice and other small rodents go looking for a warm, sheltered place. A parked car with a warm engine and a pile of leaves under the hood checks every box.

Rodents often build nests in sheltered spots under the hood, including inside the air filter housing, and they like to chew. Chewed wiring and nesting material in the wrong place can lead to warning lights, electrical problems, and repairs that are a lot more frustrating than a few leaves.

Warning signs:

• Leaves, grass, insulation, or shredded material in odd places under the hood
• Droppings on top of the engine or in the air filter housing
• A bad smell from the vents when the fan comes on
• New warning lights or electrical glitches, especially on a vehicle that sits a lot

If your vehicle is parked outside most of the winter, or sits for long stretches, it's worth a look under the hood every so often.

5. Don't Park on a Pile of Dry Leaves

One more fall safety note. Your exhaust system and catalytic converter run very hot. Parking or idling over a deep pile of dry leaves puts that heat right next to something that burns easily. Rake them out of the driveway, or park on clear pavement.

6. What You Can Do in Your Driveway

You don't need to be a mechanic to stay ahead of this:

• Every week or two in the fall, look at the base of your windshield and brush or pick out the leaves you can reach.
• Pop the hood and look around the cowl and the air intake for packed leaves or nesting material.
• If you see water inside the car, fogging that won't clear, or a musty smell, don't wait. The longer water sits, the bigger the job.
• If you find a nest or chewed material, have the wiring and air filter checked before winter.

Don't pull panels off or poke around near moving parts with the engine running. If the cowl is packed deep or you see signs of water inside, let us take a look.

The "Longmont Factor"

Northern Colorado fall makes this worse than people expect. Big, mature trees line a lot of our neighborhoods, the wind off the foothills scatters leaves everywhere, and a lot of vehicles in Longmont live outside in driveways and on the street. Then the first snow lands on top of those packed leaves, melts in the afternoon sun, and refreezes overnight, which is exactly when clogged drains cause trouble. And as the temperature swings from a 70-degree afternoon to a 25-degree night, critters start looking for warm places to spend the winter.

Diagnose First. Then Decide.

A musty smell, a foggy windshield, or a damp floor can come from a few handfuls of leaves or from something bigger, like a leaking seal or a heater core. Guessing gets expensive. At our clean shop at 110 East Ken Pratt Blvd, our techs look under the hood, find the real cause, and text you photos and findings through our Digital Vehicle Inspection (DVI), so you see exactly what we see before you decide anything.

The Nice Difference Experience:

• Transparent Results: We separate what's urgent from what can wait, so you stay in control of your budget.
• Free Courtesy Inspection with any service: Includes a cabin air filter check and a coolant check, plus a free battery test, which is perfect timing before winter.
• Zero Disruption: Drop your car off with Bryan, Josh, Mark, or Bob at the front counter and ride home or to work on our Local Courtesy Shuttle.
• Nationwide Peace of Mind: Qualifying repairs are backed by our industry-leading 3-year/36,000-mile nationwide warranty (whichever comes last).

Leaves belong on the lawn, not under your hood. Call Josh, Lindsy, and the team today at 720-605-0932, or book online at scheduler.cbac.com/longmont!