If your air conditioner smells, it is likely that there is an electrical problem with the unit. If you notice any smoke or sparks coming from the unit, immediately turn it off and unplug it, especially if it smells like burning plastic.
Do not attempt to repair the unit yourself; call a qualified technician to inspect it and make any necessary repairs not necessarily mean an expensive repair but it does need to be checked out. Attempting to troubleshoot through-the-wall air conditioners especially, would be a challenge.
It could be something as trivial as dirty air filters, though it could be a ruptured sewer vent as well. The following are different air conditioner smells and how you should approach the problem
Burning or Electrical Odors
Burning central air conditioner smells are a common issue indicating a mechanical issue with the air conditioning’s components.

When you first turn the air conditioner on after long disuse, it can emit a burning smell that is both electrical and dusty. After half an hour, the smell may go away which means it burned off the dust in the air conditioning unit.
However, if it continues, the problem may be serious requiring professional assistance. It would not be wise to do DIY AC repair and try to fix a problem with the fan motors, or compressor as that could result in severe injuries.
Turn off the power to the circuit from the breaker box and schedule an appointment with a regular HVAC technician.
Exhaust Fumes
If you smell gas in the house, it is not only disturbing but very dangerous. Air conditioners tend to run on electricity rather than gasoline so exhaust fumes are a sign the fluids within the motors are leaking. It could also be a natural gas leak.
It may not be possible to find the cause of the issue through a visual inspection and even if it was, proceeding with the repair is not recommended [1]. Rather contact your technician as soon as possible.
Mold or Musty Odor
If there is a lot of humidity in the HVAC system, there may be mold in the vents. Mold is a potential problem as it can lead to upper respiratory health issues if you let the issue persist.

Unfortunately, mold tends to grow easily within homes as all it needs is moisture, food, and oxygen.
A preemptive approach is then required to improve indoor air quality. One of the ways to do this is by having technicians examine the house to remove all mold sources.
Otherwise known as the Dirty Socks Syndrome, moldy smells are sometimes caused by dirty evaporator coils. You would have to schedule a repair session with an HVAC contractor.
Depending on how dirty it is, you may need to repair or replace the system’s evaporator coil. If the system is under warranty, the coverage should cover the costs of a new evaporator coil.
Rotten Egg Smell
The stench of rotten eggs has been likened to sulfur, so it is not something to get used to unless you live near a factory. However, such an organic odor could be from decomposing animals in the vents.

Sometimes, birds, squirrels, or other rodents find their way into the vents during the winter to get warm. Unfortunately, they get trapped, eventually, die and begin decomposing.
These pose a health hazard to the residents, considering the different diseases spread through decaying matter. You should check all ductwork for a dead animal, urine, and fecal matter to remove the smell.
These air conditioner smells may also be caused by taking too long to empty the garbage. That leads to odors going from the kitchen to the vents and circulating through the home.
A thorough cleaning of the home and filters should help to keep the air conditioner odors controlled.
Gunpowder Air Conditioner Smells
The smell of gun powder has a chemical and electric tinge. if the scent is distinct gunpowder-it may be the result of damaged fan motors. The metals that cause the scent are dangerous leading to potential respiratory issues.
In this case, do not attempt to fix it yourself. Rather, it would be advisable to call professional repair personnel to solve the issue.
Before you restart the new system after a long time of rest, it should be checked by a professional.
Chemical Smells
It’s not good if the air conditioner has a chemical scent when you turn it on. The scent may be sweet or like paint thinner, indicating the air conditioner suffers from a refrigerant leak. Even the quietest window air conditioners can have issues like this.
On the other hand, open chemical containers stored near vent intakes could be the source. Once they are picked in the airflow, these smells will waft through the rest of the house.
It would be prudent then to check the house for exposed chemicals. If they are not apparent, call a professional to find the source of the leak.
Tips to Stop your Air Conditioner System from Smelling Bad
Some air conditioner problems are light so the recommendation is to change the filter. that lessens the chances of electrical faults or dusty smells. Changing the filter also allows you to remove spores, fecal matter, and other unsavory materials that may be the cause of the foul smells.
That being said, replacing the air filters is not the only air conditioning system maintenance that leads to a longer-lasting HVAC system.
When troubleshooting, use your other senses as well. Check for knocking or clunking as well as liquids from a refrigerant leak. Sometimes, a smelly air conditioner does not indicate immediate danger but that the air conditioner needs service.
You need a regular tune-up for the system at least once a year and this should be done by a professional.