Professional auto technicians use their sense of smell, touch, sight and hearing all the time to help diagnose vehicle problems. With a little knowledge and practice you can as well. In this article series, “Use Your Senses to Diagnose Vehicle Problems”, we’ll explain how to use each one of your senses as a diagnostic tool. Let’s start with the sense of smell.

What’s that Smell?

The sense of smell can vary from one person to the next, but the odors vehicles emit are usually distinct and easily identified. If you get a whiff of an unusual smell coming from your vehicle, don’t ignore it. Here are eight of the most common automobile smells. You can use them as a guide to sniff out problems like a pro.

Musty Basement

If your vehicle smells like a damp basement or an old gym sock when using either the heater – or most commonly – the air conditioning (A/C) system, it’s likely mold and mildew has grown in the A/C evaporator case, ducting or blower case.

First, let’s discuss why it happens. During normal operation, your vehicle’s A/C pulls moisture from the air as it travels through the A/C evaporator, which is located under the dashboard. The water that accumulates from that process exits the vehicle through an evaporator case drain hose.

Sometimes that drain hose gets partially or completely clogged with leaves, dust, pollen and other debris that enters the A/C evaporator case through the fresh air intake vents. That debris mixed with the accumulation of standing water in the evaporator case creates mold and mildew. Turning on the blower helps the spread the mold spores into the entire heating and A/C system.

Carpet and floor mats that repeatedly get wet also create a perfect environment for mold and mildew to grow. If your carpets feel damp or wet, look for water entering the interior from leaking door, trunk or windshield seals or malfunctioning evaporator or sunroof drains. The sooner you fix the leak, the better.

Burnt Plastic or Electrical

Electrical wiring’s insulation can rub through so that bare wires touch grounded metal or each other, which causes a short circuit. Usually, a fuse blows to protect the rest of the circuit – sometimes not. Electrical components can also short out internally and melt. Bare wires and short-circuited components are the most common cause of electrical fires.

Another common source of a burnt plastic smell can be a plastic bag or a water bottle cooking on the exhaust system.

Burnt plastic and electrical smells warrant an immediate and safe exit to the side of the road.

For electrical issues, be sure to disconnect the negative (-) battery cable from the battery and wait until things cool down before making repairs.

Separate burned wires from each other and any metal they may be touching, then insulate them with a non-metallic material. Electrical tape is best. But for a temporary repair, you can use something like medical tape or adhesive bandages from a first aid kit. When the problem is a burnt electrical part, unplug it.

Before you get back on the road, be sure your lights work (especially brake lights), and that the engine runs normally, especially if you had to unplug an electrical part.

Rotten Eggs or Sulfur

A rotten egg or sulfur smell with the engine running is typically due to a problem with the catalytic converter. One of its functions is to convert hydrogen sulfide into sulfur dioxide. When it’s not working properly or too much fuel is going into the engine, the rotten egg smell is the result of hydrogen sulfide gas coming out the tailpipe instead of the odorless sulfur dioxide gas.

It’s best to get the vehicle to a shop for comprehensive diagnosis as soon as possible. Even if it’s too late to save the catalytic converter, and it needs to be replaced, be sure the shop diagnoses and corrects the reason why the converter went bad in the first place. Otherwise, you could be back in a few months for another catalytic converter.

Sweet (Maple Syrup)

A sweet smell inside the vehicle may indicate a leaking heater core, heater hose or hot water valve. Antifreeze (coolant) leaking from a heater core may also cause the inside of the windshield to fog up when you turn the defrosters on, or the vehicle sits overnight. It’s not uncommon for you to smell a small heater core leak even before the carpets get wet or the windshield fogs.

If you smell something sweet outside the vehicle and mostly when you start your engine (especially in the morning), antifreeze could be leaking from your engine’s cooling system. A sweet-smelling white smoke coming out of the exhaust pipe indicates antifreeze is leaking into the engine’s combustion chamber, usually through a bad head gasket. Antifreeze can also drip onto a hot surface from a leaking hose, radiator, water pump or thermostat gasket. If you see a puddle of antifreeze under the vehicle, open the hood and get under the vehicle as well to do a complete inspection.

Driving your vehicle with a coolant leak could cause the engine to overheat. If you can’t make the repairs yourself, take your vehicle to the nearest repair shop as soon as possible.

Burnt Oil

Vehicles don’t really leak oil these days with the auto industry’s use of better sealants and gaskets. But, if you do smell oil burning, check the engine oil level on the oil dipstick first. In time, even a small engine oil leak can drop the oil level enough to cause engine damage.

Most leaks will produce a burnt oil smell. How easy it is to smell usually depends on what surface the oil is leaking on. If it’s leaking on a hot exhaust manifold, you’ll easily be able to smell it, even on the freeway. The smell may also be accompanied by smoke coming out from under the hood. If oil is leaking onto the engine block, you should be able to smell it with the windows down sitting at a stop light or when you park the vehicle and get out.

Look for oil leaks under your vehicle when it’s parked. Engine splash guards may keep a small oil leak from reaching the ground so you may have to remove it to do a thorough inspection. Sources of engine oil leaks you can usually see from under the vehicle are, front or rear crankshaft seals, head gasket, oil filter, oil pan gasket or drain plug. Also inspect the top of the engine for oil leaks. Valve cover gaskets, oil fill cap seals and PCV valve grommets are notorious sources of oil leaks that you can see from up top.

If the engine oil level is correct and you can’t find any engine oil leaks, check your transmission fluid level. Automatic or manual transmission fluid can blow onto the exhaust pipe while driving and cause a strong oil burning smell.

Oil leaks like coolant leaks need to be fixed as soon as you notice them. Fix leaks yourself or take your vehicle to the shop as soon as you can. If you see smoke coming out from under the hood while driving, it’s recommended to pull over immediately and have the vehicle towed home or to a shop.

Gas or Fuel

Officially, lack of maintenance is the #1 cause of vehicle fires, but fuel leaks rank at the top of that category. A fire could start if fuel contacts the hot exhaust system or hot engine components. If you smell gas and you’re not at the gas station, always investigate the source of the leak before you start or restart the engine. In some cases, a loose or missing gas cap can be the culprit, so always check that first, especially if you just left the gas station.

Other common sources of fuel leaks are broken or split fuel lines, injectors or a leaking fuel pressure regulator. Look for leaks under the hood, all fuel lines and the gas tank.

Another possible source of fuel smells can be caused by a malfunctioning evaporative emission control system. For instance, a bad purge valve can cause fuel to saturate the evaporative cannister. Because of the dangerous consequences surrounding fuel leaks, it’s best to tow the vehicle to the shop for repairs, especially if you see no obvious problems that you can fix yourself.

Burnt Rubber

If you’ve ever been to a drag race, drifting competition or had your vacuum cleaner belt start slipping, you have a good idea of what burning rubber smells like. A burning rubber smell from your vehicle can be caused by a slipping engine belt (usually accompanied by a loud screeching noise under the hood) or an engine belt rubbing on a radiator hose or power steering hose.

When you smell burning rubber while driving, stop as soon as safe to do so, open the hood and inspect the belts and hoses for damage. If you cannot repair the issue on the spot, call a tow truck to take you to the nearest shop. You don’t want to risk overheating the engine because of a broken drive belt or leaking coolant hose.

Burnt Carpet

An over-heating catalytic converter can cause the carpet to smolder. If you smell hot carpet, pull over immediately and look under the vehicle. In most cases, you’ll be able to see the catalytic converter glowing red. If it is, cautiously use your hand to feel for a hotspot on the carpet above the catalytic converter. If it’s warm or hot, you have verified the problem.

When a catalytic converter overheats, it’s best to tow the vehicle to the nearest shop for diagnosis. Overheating can quickly destroy an expensive catalytic converter. Be prepared to also have the engine diagnosed to find the root cause of the catalyst overheat condition.

Bonus! – New Car

Let’s conclude with a pleasant smell that almost everyone likes – That New Car Smell. It’s probably no surprise that the materials that make up the interior such as leather or cloth upholstery, plastic interior panels and trim, carpet, resins and adhesives are what generate that lovely fragrance. Unfortunately, many of the things that generate that new car smell can contain volatile organic compounds (VOCs), which can be potentially hazardous to your health. Luckily, auto makers have been continuously making great progress toward eliminating VOCs in vehicle interiors.

Use Your Senses to Diagnose Vehicle Problems
Sense of Touch | Looks Like Trouble | Did You Hear That?