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Car repair glossary

The words you'll hear at the service counter, explained the way a tech would explain them if they had a minute.

Aftermarket parts: Parts made by a company other than the carmaker or its original supplier. Quality ranges from excellent to junk, so the brand matters more than the label. More →
Air-oil separator (AOS): A part on many Porsche engines (and some others) that separates oil mist from crankcase gases before they go back into the intake. When it fails you can get smoke at startup, a rough idle or oil getting sucked into the engine. More →
Angle gear: Also called the bevel gear or power transfer unit. On AWD Volvos it sends power to the rear wheels. It can leak or run low on fluid. More →
Balance shaft: A weighted shaft that spins to cancel engine vibration. On some Mercedes M272 V6 engines the gear that drives it wears out and throws off cam timing. More →
Book time: The standard number of labor hours a repair guide lists for a job. Most shops bill book time times their hourly rate, not the actual time on the clock. More →
Brake fade: When brakes get so hot they stop working as well, usually on a long downhill. Old fluid and worn pads make it worse. Use a lower gear coming down I-70.
Brake fluid flush: Replacing old brake fluid with new. Brake fluid absorbs moisture over time, which lowers its boiling point. Matters a lot on long mountain descents. More →
Carbon buildup: Hard deposits on the intake valves of direct-injection engines. It can cause rough idle and misfires and is removed with walnut blasting. More →
Check engine light: The amber engine symbol on the dash. Steady means get it scanned soon. Flashing usually means an active misfire, so ease off and get it looked at right away.
Condition Based Service (CBS): BMW's system that tracks oil, brakes, brake fluid and other items and tells you when each is due, rather than using a fixed schedule. More →
Coolant (antifreeze): The fluid that carries heat away from the engine. Different brands need different types, so use the spec your car calls for rather than a universal one. More →
CVT: Continuously variable transmission. Uses a belt or chain between two variable pulleys instead of fixed gears. Common on Subaru, Nissan, Honda and Toyota. Fluid condition matters a lot. More →
Dealer: A shop owned by a franchised new car dealership. Usually higher labor rates than independents, with factory training and tools. More →
Direct injection (DI): Fuel sprayed straight into the cylinder at high pressure instead of into the intake port. Better power and economy, but it leads to carbon buildup on intake valves. More →
DSG: Volkswagen's dual-clutch automatic transmission. The wet-clutch versions need a fluid and filter service on a schedule, usually around 40,000 miles. More →
Expansion tank: The plastic reservoir that holds extra coolant and lets it expand when hot. On European cars these get brittle and crack with age. Never open it hot. More →
Gasket: A seal between two engine parts. Rubber gaskets harden with heat and age, which is why older engines start leaking oil. More →
Head gasket: The seal between the cylinder head and the engine block. When it fails it can leak oil and coolant outside or let combustion gas into the cooling system. More →
High-pressure fuel pump (HPFP): The engine-driven pump that raises fuel pressure for direct injection. On the BMW N54 it had a reputation for failing. More →
IMS bearing: The bearing that supports the intermediate shaft on Porsche M96 and early M97 engines. Failure can destroy the engine, so many owners retrofit an upgraded bearing. More →
Independent shop (indie): A repair shop not tied to a dealership. Specialists focus on one or a few brands. Labor rates are usually lower than dealers. More →
Interference engine: An engine where the valves and pistons share space at different times. If the timing belt or chain fails, they can collide and cause major damage. More →
Labor rate: What a shop charges per hour of labor. Multiply by book time to get labor cost on most quotes. More →
Limp mode: A protective mode where the car limits power or gears after detecting a serious fault, so you can get off the road safely.
Maintenance Minder: Honda and Acura's system that calculates when service is due and shows codes like A or B plus numbers for extra items. More →
Mechatronic unit: The electronic and hydraulic control unit inside a DSG transmission. Expensive to replace, which is one reason the fluid service matters. More →
Misfire: When a cylinder doesn't fire properly. Common causes are spark plugs, ignition coils, injectors, vacuum leaks and carbon buildup. More →
OE parts: Original equipment supplier parts. Made by the same company that supplies the carmaker, sold in their own box. Often the same part as the dealer's, for less. More →
OEM parts: Parts sold in the carmaker's own branded box, usually through a dealer. Usually the most expensive option. More →
Oil filter housing gasket: The seal between the oil filter housing and the block on many BMWs. When it leaks, oil can soak the serpentine belt. More →
PCV valve: Positive crankcase ventilation valve. Routes crankcase gases back into the intake. When it fails you can get oil leaks, whistles, rough idle and oil consumption. More →
Rear main seal (RMS): The seal where the crankshaft exits the back of the engine. Replacing it means removing the transmission, so it's often done with a clutch job. More →
Serpentine belt: The long rubber belt that drives accessories like the alternator and A/C compressor. Oil contamination ruins it fast. More →
Service A / Service B: Mercedes-Benz's alternating maintenance services. A is mostly an oil service and inspection, B adds more items. More →
Thermostat: A valve that controls coolant flow so the engine warms up quickly and then stays at the right temperature. Stuck closed means overheating. More →
Timing belt: A toothed rubber belt that keeps the crankshaft and camshafts in sync. Has a replacement interval by miles and age. More →
Timing chain: A metal chain doing the same job as a timing belt. Meant to last much longer, but tensioners and guides can wear. More →
Timing chain tensioner: Keeps the timing chain tight. Early VW/Audi 2.0T TSI tensioners could fail and let the chain jump. More →
Valve cover gasket: The seal between the valve cover and the cylinder head. A common oil leak on older engines, often dripping onto the exhaust. More →
VCM (Variable Cylinder Management): Honda's system that shuts off some cylinders on certain V6s to save fuel. Linked to oil consumption and misfires on some model years. More →
Walnut blasting: Cleaning carbon from intake valves by blasting crushed walnut shells into the intake ports with the manifold removed. More →
Water pump: Circulates coolant through the engine and radiator. Some are belt or chain driven, some (like on many BMWs) are electric. More →