A Reddit user posted a puzzling problem to the /r/Autos message board. Her fiancée’s Ram truck’s interior reeks of rotten lettuce. No, he didn’t forget a bag of groceries under the back seat. The couple could find no cause whatsoever. But the internet agrees there may be a dangerous mold infecting the car’s air conditioning system.

Here’s the wording of the original post. Perhap’s you’ve experienced a similar or lesser smell:

“My fiance has a ram 1500 Laramie and out of nowhere the inside smells like rotten lettuce!!! The smell is so strong he comes home every day smelling like it. We can’t figure it out!…We’ve checked for a bunch of things, Back window not leaking, Vents in front and between the bed, Changed the cabin air filter, Sprayed Lysol down the vents, Looked under the hood and didn’t see anything dead, Cleaned the carpets and put baking soda all over the carpets, etc. [sic]”

Reddit User

There were a couple of wisecrack answers. The most upvoted was “You’re fiance is cheating on you with Lettuce.” My personal favorite was, “Its common among the 1500 Laromaine.” But I do always love a well-tossed pun.

Thankfully, some commenters were more serious than I am. They seemed to have arrived at a consensus: “That’s not lettuce, that’s mold…”

There are a couple of likely spots for mold to gather in the HVAC system. Changing the cabin air filter is a good start. But the poster had already done that maintenance task.

The second possibility is the heater core. Its a sort of mini radiator in the HVAC system, heated by the engine coolant, and with passages for air to flow through to warm the vehicle. One commenter recommended checking there: “Probs need to disinfect the heater core. Ozone generator might be able to do that.”

I’m not saying they are wrong. Heater cores do grow mold, but I’ve never seen it on a vehicle built in the past couple decades. The liquid all stays inside the fins of the core, so it’s not a great environment for mold until the entire passenger compartment springs a leak. The vast majority of commenters seemed to agree with me.

The third possibility is the air conditioner’s evaporator. This is a reverse heater core, filled with chilly freon gas instead of hot engine coolant. Normally, air passes through the air conditioner’s evaporator, cools, and drops humidity in the process. This humidity condenses into water, and drains out the bottom of the evaporator. If you look closely, you can often see the liquid dripping from a tube beneath the passenger’s side of your car on a hot day. When this tube gets clogged, perhaps with dust from a dirt road, that water builds up. And this creates a great environment for mold.

While many commenters offered help, one summed up the problem especially succinctly: “Mold forms on the evap core of many modern cars, leading to a strong musty smell. Clean it, then remember to turn off the a/c a few minutes before you arrive at your destination so moisture isn’t trapped while the car sits.” That’s some handy maintenance help. It is also a good idea to locate the drain tube and make sure it’s draining when the A/C is turned on.

The final possibility would be the line that drains from the condenser (at the front of the car) into the evaporator.

The Centers for Disease Control warns that exposure to mold in your car’s air conditioning system is a major health risk that can cause eye and throat irritation, sneezing, and even coughing. Untreated, it can lead to inflammation of your lungs, sinusitiis, bronchitis, exacerbate existing asthma, or even cause pneumonia. So if you’re worried about your HVAC system, it’s worthing doing some maintenance before the summer hits. Luckily, there’s one quick procedure that could clean out your HVAC system, no matter where the mold is forming.

Next, learn how to adjust your tire pressure for a summer road trip, or see some tips on how to fight A/C system mold yourself in the video below: