Mechanics Reveal the 5 Check Engine Codes Drivers Ignore Most (And Why That’s a Mistake)

Auto technicians report that many drivers continue driving for weeks — sometimes months — after certain check engine codes appear.

The problem?

Some of these “ignored” codes can slowly lead to expensive damage.

Here are the five codes mechanics say drivers underestimate the most.

⚠️ 1️⃣ P0420 – Catalyst Efficiency Below Threshold

Many drivers ignore this code because:

• the car still runs fine

• no major performance loss

• only a warning light appears

However, mechanics warn that prolonged driving may:

• overheat the catalytic converter

• increase emissions

• lead to expensive replacement

💰 Replacement cost can reach $1500–4000.

⚠️ 2️⃣ P0171 – System Too Lean

Often ignored because symptoms are subtle.

Early signs may include:

• slightly rough idle

• minor hesitation

• small drop in fuel economy

If left unresolved, lean operation may cause:

• overheating

• misfires

• valve damage

⚠️ 3️⃣ P0172 – System Too Rich

Drivers sometimes dismiss this code if the vehicle feels normal.

But running rich over time can cause:

• carbon buildup

• spark plug fouling

• catalytic converter damage

Experts say fixing small airflow or injector issues early prevents larger expenses.

⚠️ 4️⃣ P0299 – Turbo Underboost

This code is frequently ignored because the car still drives — just slower.

But mechanics warn:

• boost leaks may worsen

• turbo may overwork

• full turbo replacement may follow

💰 Turbo replacement often exceeds $2000.

⚠️ 5️⃣ P0016 – Timing Correlation Error

Timing codes are sometimes ignored until noise becomes noticeable.

However, mechanics say early oil-related timing issues may escalate into:

• stretched timing chain

• misaligned cam timing

• internal engine damage

Repair costs increase dramatically if ignored.

Why Drivers Ignore These Codes

According to technicians:

• no obvious symptoms

• vehicle still drives normally

• drivers assume it’s “just a sensor”

• fear of repair costs

Ironically, delaying repairs often makes them more expensive.

How Mechanics Recommend Handling These Codes

Professionals suggest:

✔ scanning codes immediately
✔ checking live data
✔ addressing small problems early
✔ avoiding aggressive driving

Early intervention usually keeps repairs manageable.

Quick Risk Comparison

Final Thoughts

Mechanics say the biggest mistake drivers make is assuming that a “normal driving feel” means no problem.

Modern vehicles detect issues early — long before major failure happens.

Ignoring these five codes may turn moderate repairs into major expenses.