Auto repair specialists are reporting a noticeable increase in vehicles showing the P0420 error code, one of the most common emissions-related problems seen today.

The P0420 code indicates that the catalytic converter is no longer operating at optimal efficiency.

While many drivers expect major engine problems when this code appears, mechanics say the situation is often more complex — and sometimes preventable.

Not all check engine lights are equal.

Some OBD2 codes indicate simple sensor problems — while others may point to major engine or emissions system repairs costing thousands of dollars.

Understanding which codes are expensive helps drivers react early and avoid bigger damage.

Below are the 10 most expensive OBD2 codes mechanics see most often.

OBD2 codes (On-Board Diagnostics II) are diagnostic trouble codes stored by your car’s computer when it detects a problem.

Every modern vehicle monitors engine performance using sensors.
If something works outside normal limits, the system stores an error code and turns on the Check Engine Light.

These codes help mechanics quickly identify problems before serious damage happens.

Seeing the Check Engine Light often makes drivers worry about expensive repairs.

But here’s good news:

➡️ Many OBD2 trouble codes are actually cheap and simple to fix.

In many cases, the repair costs less than a normal service visit — especially if diagnosed early.

This guide covers the most common low-cost OBD2 codes and their typical repair prices.

Not every Check Engine Light means immediate danger.

Some codes are minor — like EVAP leaks — but others can quickly lead to expensive engine damage if ignored.

Below are the 7 most serious check engine codes that every driver should take seriously.

Seeing the Check Engine Light turn on can be stressful for any driver.

However, many people don’t realize there are two different types of warning signals:

• A solid (steady) check engine light

• A flashing (blinking) check engine light

Understanding the difference is extremely important — because one may be safe for short driving, while the other can quickly cause serious engine damage.

P0304 indicates misfire detected on Cylinder 4.

This is one of the most searched misfire codes because it appears frequently on 4-cylinder engines.

The Check Engine Light (CEL) is one of the most common warning lights drivers see.

It means the vehicle’s computer has detected a problem related to:

• engine performance

• emissions system

• fuel mixture

• ignition system

• sensors or electronics

In many cases, the car may still drive normally — but the warning should never be ignored.

P0303 means the ECU detected combustion problems in Cylinder 3.

Most cases are ignition-related — not major engine failure.

The Check Engine Light is one of the most common warnings modern drivers see.

While thousands of diagnostic trouble codes exist, mechanics report that a small group of OBD2 codes appears much more frequently than others.

This guide explains the 10 most common OBD2 codes, what they mean, and what usually causes them — helping car owners understand problems before expensive repairs happen.