Owner's book · not a shop

Open Hood

Know the car. Then the window. Factory specs in plain English — don't authorize until you can say this at the counter.

No car yet Try the demo · 2003 Honda Accord

Owner tools

Lab desks

No dongle required

Type the code

Any $20 scanner. Five characters. We write what it means and what to ask before parts. iPhone Safari has no Web Bluetooth — type the code.

Park first · Mode 03

ELM327

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P0420

Catalyst efficiency below threshold (Bank 1)

soon · typical $200–$2,500

The downstream O2 sensor looks too much like the upstream one. That can be a dying catalyst — or an exhaust leak, a lazy upstream sensor, or a misfire that poisoned the brick.

Usual first look · Cat, O2 sensors, exhaust leak, misfire

Ask them to verify no misfires, no exhaust leaks, and to graph both O2 sensors before quoting a converter. A converter is $800–$2,500; a sensor is not.

Say this
I have P0420 stored. I want freeze-frame and a leak check before anyone prices a converter.

Not a driveway parts-cannon — pay for the test

SAE layout: powertrain (engine / transmission). Subsystem looks like auxiliary emissions (EVAP, EGR, catalyst). Generic (SAE) code.

This is an SAE-generic layout. The number is real even if our dictionary has no paragraph yet.

Take P0420 to quote defense

Paste the RO. Circle the line they hung on this pointer. The code is not a parts list.