Car Repair Questions, Answered

Your car's on-board diagnostic (OBD-II) system found a fault, usually in the engine or emissions controls, and stored a trouble code. Common causes include a loose gas cap, a failing oxygen sensor, worn spark plugs or ignition coils, a mass air flow sensor, or a failing catalytic converter, according to CarMD's 2026 index. Only reading the code and testing the related parts tells you which.

It means the computer that monitors your engine and emissions has detected a problem and wants the car serviced. The EPA describes a steady light as a request for service, not a signal to stop the car. A flashing light is more urgent: it signals a severe misfire that can damage the catalytic converter.

Notice whether it's steady or flashing, and how the car drives. If it's steady and the car feels normal, make sure the gas cap is tight and book a diagnosis soon. If it's flashing, slow down, avoid heavy loads and get service right away. If a temperature or oil pressure warning also comes on, AAA says to pull over safely and get help.

Fix the cause and the light resets itself: the EPA says the OBD system turns the light off after three trips in a row without the fault, or a technician clears it after the repair. Clearing codes with a scan tool or disconnecting the battery only hides the problem and resets the readiness monitors, so BAR says to drive for a week or two before a Smog Check.

A flashing light means a severe engine misfire is happening right now. Unburned fuel from the misfire can overheat and damage the catalytic converter, often the most expensive part to replace, the EPA says. Reduce speed, avoid towing or hauling, and get the car serviced as soon as possible. If the car shakes or loses power, stop somewhere safe.

The misfire that caused the flashing may only happen under certain conditions, such as climbing a hill or accelerating hard, so the light can drop back to steady or go out when the load eases. The problem hasn't fixed itself. The code stays stored, and the next hard pull can set it off again, so get it diagnosed before the catalytic converter is damaged.

OBD-II rechecks faults as you drive. If it tests a component three consecutive times without seeing the problem, it turns the light off, the EPA says, and it turns the light back on if the fault returns. A light that keeps cycling points to an intermittent problem, like a weakening sensor or an occasional misfire. Ask a mechanic to read the stored code even when the light is off.

Plug an OBD-II scan tool into the diagnostic connector, which the EPA says is usually under the dashboard, and read the stored codes. Many auto parts stores will read them for free, Kelley Blue Book notes. Write down the exact code. Remember that a code names a system, not always the failed part: AAA says P0420 can mean a bad catalytic converter or a much cheaper oxygen sensor.

It's usually a yellow or amber engine outline, or the words 'Check Engine' or 'Service Engine Soon,' the EPA says. It should light briefly when you turn the key, as a self-check, then go out. Don't confuse it with the red oil pressure or engine temperature warning lights; AAA says to stop driving and get help if either of those comes on.

Many do. Kelley Blue Book notes that many auto parts stores will read your check engine codes without charging a fee. That free scan gives you the code, which is a good starting point. A repair shop's paid diagnosis goes further by testing the parts behind the code; RepairPal's national average for that service is $125 to $183.

No. The check engine light reports engine and emissions faults, not maintenance due dates. Many cars have a separate service reminder or oil-life message for that, BAR notes, so check your owner's manual. A red oil can symbol is something else again: it's the oil pressure warning, and AAA says to stop driving and get help if it comes on.

CarMD's 2026 Vehicle Health Index, based on 2025 data, ranked them: catalytic converter (average $1,511), oxygen sensor ($287), ignition coil and spark plugs ($480), mass air flow sensor ($346) and ignition coil ($256). The average check engine repair was $554. A loose gas cap is still a common, cheap cause; CarMD says tightening it is free.

Start with businesses registered with the Bureau of Automotive Repair, which you can verify in the DCA license search. Ask whether the technicians are ASE-certified, get an estimate before any work starts, and ask how long the repair is covered. Reviews and referrals help, but registration and a clear estimate protect you if something goes wrong.

A mechanic, also called an automotive service technician, inspects, maintains and repairs vehicles: diagnosing warning lights and noises, replacing brakes, batteries and belts, servicing fluids, and fixing engine, transmission, electrical and air conditioning problems. In California the repair business must be registered with BAR. Many mechanics also hold ASE certifications, which require passing a test plus hands-on work experience.

Follow your owner's manual or oil-life monitor, not the old 3,000-mile rule. CalRecycle says many cars can go 5,000 to 15,000 miles between changes. It also notes that 'normal' conditions exclude heat over 90 degrees and lots of stop-and-go traffic, both common in Sacramento summers, so your manual's severe-service schedule may be the one that fits.

At least once a year, the Car Care Council recommends, and AAA Mountain West Group says experts suggest every six months, more often if you drive aggressively. Get them checked right away if you hear grinding, the pedal feels soft or sinks, or the car pulls to one side when you stop.

When the tread wears down to 2/32 of an inch, NHTSA says. Use the penny test: put a penny in the tread with Lincoln's head upside down, and if you can see the top of his head, it's time. Also replace tires with cuts, cracks or bulges. Check pressure monthly when the tires are cold, using the number on the driver's door label, not the sidewall.

Often less than you'd expect. AAA says batteries typically last three to five years, but heat can cut that to as little as two. Kelley Blue Book says extreme temperatures can limit a battery to three or four years. Watch for a slow start, warning lights, odd smells or corrosion on the terminals, and have the battery tested before summer.

No. BAR exempts electric vehicles from Smog Check. Hybrids are treated like gasoline cars and need checks at renewal once they're more than eight model years old, and when sold if more than four model years old. EVs still need maintenance, though the Department of Energy says their brakes tend to last longer thanks to regenerative braking.

90 days. BAR says the station sends the certificate to the DMV electronically, and it stays valid for 90 days. When a car is sold privately, it must have passed a Smog Check within the last 90 days, while a car bought from a dealer must have passed within the past two years.

The seller. BAR says a Smog Check is required before you sell, and you should give the buyer a copy of the vehicle inspection report. If the car is four model years old or newer, no check is needed; the buyer pays a smog transfer fee instead. Transfers between immediate family members generally don't need a check, the DMV says.

A Smog Check station certified by BAR to meet higher performance standards. STAR stations can inspect every vehicle, including cars the state directs to them through the DMV renewal notice. If your notice says 'Smog Certification Required at a STAR station,' you have to use one. The DMV says the STAR program began January 1, 2013, replacing the old Gold Shield and Test Only program.

Yes, if you ask when you authorize the repair. California lets you request the return of replaced parts, BAR says, so make the request before work starts. Parts replaced under warranty are an exception because they usually must go back to the manufacturer, and a part sold on an exchange basis, like a battery, may carry a core charge if you keep the old one.

It protects people who buy or lease a new vehicle, or a used one still under the manufacturer's new-vehicle warranty, when a serious defect can't be fixed; the maker must replace or repurchase it. The DCA says the law presumes enough attempts if, within 18 months or 18,000 miles, the same problem survives four repairs, a dangerous defect survives two, or the car spends over 30 days in the shop.

Enter your 17-character vehicle identification number (VIN) at NHTSA.gov/recalls. NHTSA says the VIN is on the lower left of the windshield and on your registration card. BAR notes that safety recall repairs are always free, and a Smog Check inspection report also lists any open safety recalls on the vehicle.