I'm Rushit Hila, founder of YOUCANIC, an automotive education platform, engineer and ASE certified. Have been working on cars for over 20 years. Don't ignore a Check Engine Light for weeks or months just becasue the car seems to drive fine. On many vehicles, especially modern diesels for example a lit CEL can prevent critical self-maintenance like DPF regeneration. A simple issue, such as a loose gas cap, will prevent regen, which self-cleans the DPF filter and snowball into costly repairs. Scan the codes promptly, address the root cause, and keep up with basics like tire pressure and fluid checks so small faults don't escalate.
The least expensive safety upgrade is a monthly "10-minute walkaround." Check the tires when they are cold (even 1 pound per square inch below the standard pressure can decrease fuel economy by approximately 0.2 percent and warm up the tread), use a coin to measure the tread depth (replace the tires when the tread is at 2/32 inches), and look for sidewall bubbles or cupping. While checking the vehicle, check for the eyeball brake feel during a short drive (sponginess or pulling under braking are red flags) and listen for chirps that indicate the wear indicators. Open the hood and check the fluids — oil in about the center of marks or where that engine would need to be (takes only 30 seconds), oil at the reservoir line, clear brake fluid, and nothing burned out power steering/transmission fluid. Opaque headlight covers are the ninjas of issues (old plastic can block up to about 80% of light output; a $20 kit will typically clear that up). Replace the wiper blades every six to 12 months, test and load the battery before winter (cranking power can decrease by approximately 35% at 32degF).