Maintenance, buying, and ownership guides
Quick answer Octane rating measures a fuel’s resistance to engine knock (detonation). It is not a power, energy content, or cleanliness rating. The number posted on U.S. pumps is the average of two test methods: Research Octane Number (RON) and Motor Octane Number (MON), calculated as (R+M)/2. This average is called AKI (Anti-Knock Index). Common…
The Evaporative Emission Control (EVAP) system captures fuel vapors from your gas tank and engine, preventing them from escaping into the atmosphere. It stores those vapors in a charcoal canister and purges them into the engine to be burned during normal driving. When the system fails, you’ll likely see a check engine light, smell raw…
An oxygen (O2) sensor measures the unburned oxygen in your exhaust and tells the engine computer (ECU) whether the air-fuel mixture is too rich or too lean. Every modern car has at least two: one upstream (before the catalytic converter) and one downstream (after it). The upstream sensor controls fuel trim and directly affects drivability…
A catalytic converter is an emissions control device mounted in your exhaust system that converts three harmful gases—carbon monoxide, unburned hydrocarbons, and nitrogen oxides—into less toxic compounds before they leave the tailpipe. It’s valuable because the honeycomb inside is coated with precious metals (platinum, palladium, and rhodium) that make replacement expensive, and because a failed…
A sway bar (also called an anti-roll bar or stabilizer bar) is a steel torsion spring that connects the left and right sides of your suspension through rubber or polyurethane bushings. Its job is to reduce body lean in corners while minimizing the impact on straight-line ride comfort. When those bushings wear out, you will…
A control arm is the suspension link that connects your wheel hub to the vehicle frame. It lets the wheel move up and down while keeping it aligned. When the ball joint or bushing wears out, you’ll hear clunking over bumps, feel steering wander, and see uneven tire wear. Replacement typically runs $300–$800 per arm…
An anti-lock braking system (ABS) prevents wheel lockup during hard braking so you can steer while stopping. It uses electronic wheel-speed sensors and a hydraulic modulator that can pulse brake pressure up to 15–18 times per second. On dry or wet pavement this typically reduces stopping distance, and on loose surfaces like gravel or snow…
A CV (constant velocity) axle transfers engine power from the transmission to the drive wheels while allowing the wheels to steer and move up and down with the suspension. On front-wheel-drive cars—and many all-wheel-drive and rear-drive vehicles—it’s a frequent wear item. The telltale signs are clicking sounds during sharp turns, grease splattered behind the wheel,…
A torque converter is the fluid coupling inside an automatic transmission that connects engine power to the gearbox while allowing the engine to idle at a stop. It multiplies torque for takeoff, smooths power delivery, and uses a lock-up clutch at cruising speeds to eliminate slip and improve fuel economy. If your transmission shudders, slips,…
The PCV (Positive Crankcase Ventilation) valve is a small one-way valve that routes combustion blow-by gases from the engine crankcase back into the intake manifold to be reburned. When it fails, you get rough idling, increased oil consumption, oil leaks, or a check-engine light. A stuck-closed valve is the worst case: rising crankcase pressure can…