Top 7 Check Engine Light Codes You Should NEVER Ignore

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.

1️⃣ P0300 – Random / Multiple Cylinder Misfire

Why it’s dangerous:
Misfires allow unburned fuel into the catalytic converter.

This can cause:

• catalytic converter failure

• severe engine vibration

• loss of power

⚠ If the check engine light flashes — stop driving immediately.

2️⃣ P0016 – Camshaft / Crankshaft Correlation

Why it’s serious:
This code often indicates timing chain problems.

Possible consequences:

• engine won’t start

• valve damage

• complete engine failure

Repairs can be very expensive if ignored.

3️⃣ P0017 – Timing Correlation Error (Bank 1 Sensor B)

Similar to P0016 but often related to exhaust camshaft timing.

Common causes:

• stretched timing chain

• VVT actuator issues

• oil pressure problems

Driving long-term may cause internal engine damage.

4️⃣ P0299 – Turbocharger Underboost

Why drivers ignore it (but shouldn’t):

The car usually still drives — just slower.

But continued driving may lead to:

• complete turbo failure

• oil leaks

• expensive replacement costs

5️⃣ P0420 – Catalyst System Efficiency Below Threshold

At first, symptoms are often small.

But if ignored:

• emissions increase

• performance drops

• converter replacement becomes necessary

Catalytic converters are one of the most expensive emissions components.

6️⃣ P0171 – System Too Lean

Danger factor: Moderate to high.

Running lean means:

• higher combustion temperatures

• potential valve damage

• increased misfire risk

Common causes:

• vacuum leaks

• MAF sensor issues

• fuel system problems

7️⃣ P0172 – System Too Rich

Opposite of P0171.

Too much fuel can cause:

• carbon buildup

• spark plug fouling

• catalytic converter failure

Fuel economy also drops significantly.

Quick Danger Comparison

What To Do If You See One of These Codes

Recommended steps:

1️⃣ Scan the code immediately
2️⃣ Check if the light is flashing
3️⃣ Avoid aggressive driving
4️⃣ Diagnose before replacing parts

Early diagnosis can save thousands in repair costs.

Why Modern Cars Show More Serious Codes

Today’s vehicles use:

• tighter emissions monitoring

• advanced sensors

• precise fuel control

Even small problems now trigger warning lights early.

FAQ 

What is the most dangerous check engine code?

Misfire codes (P0300 series) are among the most dangerous if ignored.

Can I drive with these codes?

Some short-term, but flashing warnings mean stop driving.

Will the check engine light go away by itself?

Sometimes, but the underlying issue usually remains.

Final Thoughts

Not all check engine lights are equal.

Understanding which codes are serious helps drivers avoid major engine damage and expensive repairs.

If you see any of the codes above — diagnose early and repair before the problem gets worse.