5 check engine codes mechanics charge $100+ to diagnose that you can read yourself

The check engine light comes on and suddenly you're looking at a $100–$150 diagnostic fee just to find out what the code is. I've been there. After picking up a basic OBD2 scanner a few years ago, I realized most of these "diagnostic appointments" are just a tech plugging in a scanner for two minutes to read a code any of us can pull ourselves in the driveway.
P0420 — Catalyst System Efficiency Below Threshold. One of the most common codes out there, especially on higher-mileage cars. It almost always means the catalytic converter is aging or failing. Shops will charge $100+ to confirm what a $30 Bluetooth adapter already told you. Knowing the code in advance means you can research fair repair pricing and shop around — not get handed a $1,200 invoice the moment you walk in blind.
P0171 — System Too Lean (Bank 1). This one can trip people up because the cause ranges from a vacuum leak to a dirty MAF sensor to a clogged injector. But pulling the code yourself first lets you research before you're stuck in a waiting room. On a Toyota Camry I helped a friend diagnose, it turned out to be a cracked intake boot — $18 part, 20-minute job. The shop quoted $300 "to investigate."
P0300/P0301-P0308 — Random or Cylinder-Specific Misfires. Misfire codes are everywhere. A scanner with live data will show you which cylinder is misfiring, which instantly narrows it to the spark plug, coil, or injector for that specific cylinder. That information alone cuts your diagnostic time in half and lets you order the right part before touching anything.
P0442 — Small EVAP Leak. Nine out of ten times, this is a loose gas cap. Tighten it, clear the code, drive a few days, see if it returns. If it does, then you have a real leak worth chasing. Don't pay $100 to be told to tighten your gas cap.
P0128 — Coolant Temp Below Thermostat Regulating Temp. Usually a stuck-open thermostat — a $15–$30 part and a straightforward DIY job on most cars. Knowing this code in advance means you walk into the parts store or the shop knowing exactly what needs to be replaced, and you're not flying blind on price.
A basic Bluetooth OBD2 adapter runs $20–$40 and works with free apps on your phone. The diagnostic fee you're trying to avoid costs 3–5× the adapter. Once you own one, you'll use it constantly — not just for check engine lights but for live data, resetting service lights, and verifying a repair actually fixed the problem.