Today’s cars, trucks and SUVs are loaded with technology, none the least of which is an onboard diagnostic (OBD) feature that will alert a driver when a mechanical problem exists with the engine or emissions system.

That’s when the feared “Check Engine” light illuminates on a vehicle’s instrument panel. With the average vehicle age on U.S. roads now up to nearly 13 years old, most motorists either have or will confront this important warning, which typically manifests as either the side outline of a car’s engine, or simply the words “Check Engine.”

While it’s prudent to make an appointment with a trusted mechanic to have the problem investigated in the near future, should the Check Engine light be flashing – especially when accompanied by the temperature or oil warning – that indicates a serious problem that should be taken care of immediately to avoid the need for further and costlier repairs.

While the warning light itself will not reveal exactly what the problem is, this is accomplished via a device called a code reader that plugs into a vehicle’s OBD port (which is usually located in the lower driver’s side of the dashboard). A local auto parts store will often perform a scan at no cost via a hand-held unit (also available for purchase) but a modern repair shop likely has a more-sophisticated diagnostic tool. That’s essential because today’s cars are packed with an array of complex computerized sensors.

As the name implies, this device will upload one or more codes that refer to a corresponding issue for that specific make and model. This can be something as simple as having a loose or defective gas cap that could cost little or nothing to fix, or it might indicate the need for a more-serious repair that can challenge one’s bank account.

Components like oxygen sensors, ignition coils, thermostats and air-flow sensors tend to fail over time, triggering dashboard lights and often adding up to more-complex repairs if neglected.

Yet sources suggest that as many as half of all motorists just ignore this important alert to delay a subsequent repair bill and keep on driving, especially if there’s no noticeable lack of performance. It’s easy to see why.

According to the annual Vehicle Health Index issued by the auto service website CarMD.com, vehicle repair costs increased by a whopping 33% last year, averaging $554. Replacement parts are, on average, 23% costlier, due largely to the proliferation of complex computerized systems, inflation and the recent round of tariffs on imported parts. Labor costs went through the proverbial roof, at a national 51% boost.

Still, it’s foolish not to have a vehicle checked out by a mechanic and have the issue rectified. For starters, it could result in as much as a 40% reduction in fuel economy, as the engine’s electronics help to compensate for what could otherwise require a minor fix (like a bad spark plug). At today’s inflated gas prices, that alone should warrant a trip to the shop.

Even worse, neglect could eventually lead to a much costlier repair. For example, a car’s catalytic converter can fail prematurely if other mechanical woes, like driving with a bad oxygen sensor or ignition coil, are ignored.

In fact, the most frequent Check Engine repair these days, according the CarMD, is to replace the catalytic converter, a critical vehicle emissions component that now costs an average $1,511 in parts and labor. However, not all converter replacements are due to component failure, as many continue to be stolen for the precious metals used to make them work.

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The website also reports that warning light alerts for problems with advanced auto safety systems are on the rise, with the ABS (antilock brakes) wheel sensor symbol reaching the top 10 list for the first time this year, which signals increased braking and traction control woes in the nation’s aging fleet.

Most-Frequent Check Engine Light Causes And Repair Costs

These are the 10 most common diagnostic-light warnings identified by the 2026 CarMD Vehicle Health Index, which is based on a study of over 39 million vehicles’ service records during calendar year 2025. Problems are noted along with their identifying symptoms and national average parts and repair costs; charges will vary due to local parts costs/availability for specific models and prevailing wages for auto mechanics:

Catalytic Converter: Symptoms include reduced acceleration, sluggish performance, dark exhaust smoke and excess engine heat; $1,511 average repair cost
Oxygen (O2) Sensor: Diminished fuel economy; $287
Ignition Coil And Spark Plug(s): Sluggish acceleration, poor fuel economy, misfires, difficulty starting; $480
Mass Air Flow Sensor: Stalling, hesitation during acceleration, poor fuel economy; $346
Ignition Coil: Rough idling, difficulty starting, could eventually affect other components, including the catalytic converter; $256
Evaporative Emissions (EVAP) Purge Control Valve: Rough idling, poor fuel economy; $172
Fuel Injector(s): Misfiring, poor idling, reduced fuel economy; $572
Antilock Brakes Wheel Speed Sensor (ABS Light): Issues maintaining traction over slippery roads, pulsating or vibration when applying the brakes, inaccurate speedometer readings, eventual ABS failure; $314
Thermostat: Overheating, failure to reach operating temperature; $324
Spark Plug(s): Misfires, diminished fuel economy performance, poor cold starting: $299