OBD-II Code Library
Plain-English answers for the most-searched check engine codes. Every page reviewed by working mobile mechanics — what the code means, whether you can drive, and what it actually costs to fix.
P0420 · Emissions
Check Engine Light P0420 — Catalytic Converter Warning
Safe to drive
You can keep driving short-term. The car runs normally; you'll only fail an emissions test until it's fixed.
P0300 · Ignition
P0300 Random Misfire — What It Means & When It's Serious
Drive cautiously
Drive short distances only. Misfires dump raw fuel into the catalytic converter and can damage it within days under hard driving.
P0171 · Fuel
P0171 System Too Lean — Common Causes & Fixes
Drive cautiously
Safe for short trips. A lean condition can damage the engine over weeks if ignored, especially under load (highway, towing).
P0455 · Emissions / EVAP
P0455 EVAP Leak — Usually Just Your Gas Cap
Safe to drive
Completely safe to drive. EVAP is an emissions system — the car runs normally, and the most common fix is tightening your gas cap.
P0128 · Cooling
P0128 Engine Coolant Won't Heat Up — Usually a Stuck Thermostat
Drive cautiously
Safe to drive but fix it soon. A cold-running engine hurts fuel economy 10–15% and increases wear over months.
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