Spanish mechanic Juan José warns that a common mistake involving car air conditioning can affect comfort and safety. Unpleasant odors may signal moisture buildup in the system, while faults in engine components or the exhaust system, along with damaged seals, can allow harmful gases into the cabin, with potentially deadly consequences, as reported by elespanol motor.
For Juan José, unpleasant odors are more than a nuisance: they can point to moisture inside the air-conditioning circuit. That moisture can encourage bacteria and mold to grow on HVAC components, including the evaporator or cabin filter. To limit musty smells, he recommends switching off the air conditioning before turning off the engine, then letting the fan run for a few minutes with outside air. This can help dry the system. Cleaning or deodorizing the HVAC system, but, addresses odors and microbial contamination, not an exhaust leak or carbon monoxide.
If an odor has already set in, Juan José advises using a bactericidal product, available as a spray, liquid or gas-based treatment, to tackle its source. In his social media videos, he demonstrates these solutions and emphasizes regular cleaning. A persistent exhaust-like smell, or unexplained headache, dizziness, weakness, nausea, confusion or unusual drowsiness while driving, calls for a different response: leave the vehicle and move into fresh air, then arrange a professional inspection.
The risks go beyond bad smells. A fault in the exhaust system, a damaged body seal or a ventilation problem can let exhaust gases enter the passenger compartment. Carbon monoxide (CO), a gas produced by motor vehicles, is colorless and odorless. The U.S. Centers for Disease Control and Prevention warns that “carbon monoxide is an odorless, colorless gas that can kill you.” Smell alone cannot establish whether cabin air is safe. The CDC also advises never running a vehicle inside an enclosed or partially enclosed space.
Vehicle emissions testing shows that CO is present in exhaust, but tailpipe figures do not measure cabin air. The UK Department for Transport’s Vehicle Market Surveillance Unit tested 18 vehicles in its 2025 emissions programme. One reported WLTC result was 377.30 milligrams of CO per kilometre, against a legislative limit of 1,000.00 mg/km. These figures describe emissions under specified test conditions. They do not show whether exhaust is entering a particular vehicle’s cabin or whether its occupants are safe.
To reduce this danger, Juan José suggests briefly opening the hood before starting the car, especially if it has been parked for a while. He also cautions against using recirculation mode while stationary. These are his recommendations. Suspected exhaust intrusion should be treated as a vehicle-safety issue, not just an odor problem. A gas detector may provide an additional warning, but it cannot replace professional checks of the exhaust system and body seals, along with an inspection of the ventilation intake.
Concerns about air quality and vehicle safety are not new. Debates over physical versus digital controls in modern vehicles show how small design details can affect driver well-being. For example, some brands still prefer traditional buttons, as explored in a recent analysis of Renault’s approach to interior safety, where practical choices can outweigh trends when protecting occupants.
Keeping the air inside a car healthy takes more than regular servicing. Drying and cleaning HVAC components can help address musty odors, while any suspected exhaust leak calls for prompt inspection. As vehicles become more complex, drivers need to distinguish routine air-conditioning maintenance from a potentially dangerous exhaust problem to protect comfort and safety on the road.
