Check-engine light on? Read the code before you pay for a diagnosis
⏱ 6 min read · 💲 $20 OBD-II reader · Updated 2026-09-27 ·
A $20 reader tells you what a $120 diagnostic fee would. Here are the six most common codes, what they mean and what the fix usually costs.
Step 1: plug in a reader
Every car since 1996 has an OBD-II port under the dash. A basic Bluetooth reader costs $15–$30 and pairs with a free phone app. Many auto-parts stores also scan for free.
The six most common codes
| Code | Meaning | Common fix | Typical cost |
|---|---|---|---|
| P0420 | Catalyst efficiency below threshold | O2 sensor or catalytic converter | $150 / $900–$2,500 |
| P0300–P0308 | Misfire (cylinder #) | Spark plugs, coil pack | $120–$400 |
| P0171 / P0174 | System too lean | Vacuum leak, MAF sensor, fuel pump | $100–$600 |
| P0455 / P0442 | EVAP leak (large / small) | Gas cap, purge valve | $15–$250 |
| P0128 | Coolant below thermostat temp | Thermostat | $150–$350 |
| P0401 | EGR flow insufficient | Clean or replace EGR valve | $150–$500 |
Flashing check-engine light = active misfire that can destroy the catalytic converter. Stop driving and get it towed or seen the same day.
Step 2: don't just clear it
Clearing the code without fixing the cause also erases the readiness monitors — you'll fail an emissions test until they reset.
Get 3 repair quotes with your code →
Affiliate disclosure: product links on this page may earn us a commission at no cost to you. It funds free guides.