The 10 Most Common OBD-II Codes and What They Cost to Fix

OBD-II has been standard on all US cars since 1996, and the same handful of codes accounts for the majority of check engine lights that come through repair shops. Knowing what each one means before you call a mechanic puts you in a much better position to get a fair price.

1. P0171 and P0174: System Too Lean

These two codes mean the engine is getting too much air relative to fuel. P0171 is Bank 1 (the side with cylinder 1), P0174 is Bank 2. The most common culprits are a dirty or failing mass airflow sensor, a vacuum leak somewhere in the intake, a clogged fuel filter, or a weak fuel pump.

A dirty MAF sensor is often the cheapest fix: a $10 can of MAF cleaner and 15 minutes of your time. If it is a vacuum leak, a shop will charge $80 to $200 to find and seal it. A fuel pump replacement runs $300 to $700 depending on the car.

2. P0300: Random Misfire

P0300 means the engine is misfiring across multiple cylinders without a clear pattern. Start with the basics: spark plugs and ignition coils. On most modern cars, a full set of plugs costs $50 to $120 in parts and can be done at home. If a specific coil has failed, count on $150 to $400 installed per coil.

If plugs and coils check out, the problem may be in the fuel injectors or a compression issue. Injector cleaning costs around $100 at a shop. Low compression means a deeper engine problem and a bill that starts at $1,000.

3. P0420 and P0430: Catalyst Efficiency Below Threshold

P0420 (Bank 1) and P0430 (Bank 2) are among the most misdiagnosed codes on the road. Mechanics often jump straight to a catalytic converter replacement at $800 to $2,500, but a failing rear oxygen sensor can trigger the exact same code for about $150 to $350. An exhaust leak upstream of the sensor can also cause it.

Before agreeing to a cat replacement, ask the shop to monitor live O2 sensor data. If the rear sensor is still cycling, the converter is likely fine. We have a full breakdown of P0420 diagnosis steps here.

4. P0455: Large EVAP Leak

The evaporative emission system captures fuel vapors so they do not escape into the air. A P0455 means there is a significant leak somewhere in it. The most common cause: a loose or damaged gas cap. Try tightening it and driving for a day or two. If the light clears, you are done. A new gas cap costs $20 to $50.

If the cap is not the problem, a shop will run a smoke test to find the leak. EVAP hoses and purge valves run $100 to $400 to fix. A cracked charcoal canister is on the higher end at $200 to $600.

5. P0456: Small EVAP Leak

Same system as P0455, but the leak is small enough that the smoke test can take longer to find it. The gas cap is still the first thing to check. Beyond that, the leak could be a cracked hose fitting or a failing vent valve. Expect $150 to $500 for a shop diagnosis and repair.

6. P0128: Coolant Temperature Below Thermostat Regulating Temperature

This code shows up when the engine is not reaching its normal operating temperature within a set time. The thermostat is usually the cause. It is a cheap part ($15 to $40) but some engines bury it in a spot that takes an hour or more to access. Total repair cost is typically $150 to $350.

Ignoring this code is not a good idea. An engine running too cool burns more fuel and wears faster than one at proper temperature.

7. P0401: Insufficient EGR Flow

The exhaust gas recirculation system routes a small amount of exhaust back into the intake to lower combustion temperatures and reduce NOx emissions. P0401 usually means the EGR valve is clogged or stuck. On some cars you can clean the valve yourself with throttle body cleaner. A replacement EGR valve costs $200 to $600 installed.

8. P0335: Crankshaft Position Sensor Circuit Malfunction

The crankshaft position sensor tells the ECU where the engine is in its rotation. When it fails, you may notice hard starting, stalling, or the engine refusing to start at all. The sensor itself costs $40 to $100. Labor varies considerably depending on where it sits on your engine. Budget $200 to $450 for a shop repair.

9. P0700: Transmission Control System Malfunction

P0700 is a general flag that tells you to look for additional transmission-specific codes. On its own it does not tell you what is wrong. Pull all stored codes before doing anything else. The transmission control module may be storing a much more specific fault alongside it. Diagnosis alone can cost $100 to $150, with repairs ranging from a $50 solenoid to a full transmission rebuild.

10. P0113: Intake Air Temperature Sensor High Input

The IAT sensor measures the temperature of air entering the engine. A failed sensor causes the ECU to miscalculate fuel delivery. The sensor is usually inexpensive ($20 to $60 for the part) and is often part of the MAF sensor assembly. Total repair cost: $80 to $250 depending on where it lives on the engine.

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A Note on Repair Costs

The ranges above reflect typical 2026 US shop rates and online part prices. Your actual cost depends on the make, model, year, and your location. Independent shops generally charge 20 to 40 percent less than dealers for the same work.

For any code, look it up on CarCodeHub first. Each code page shows you the most likely causes ranked by frequency, a severity rating so you know how urgently you need to act, and DIY-vs-shop cost estimates for that specific code.