The check-engine light has a name
Why the light is on.
The check-engine light is on. Look up the code before you assume it is fine. The car already stored a short code that names the problem. The dashboard will not say that name. WhyLamp will.
Why this exists
Your car already knows why the light is on. It will not say it out loud.
Today’s cars run tests while you drive. When a test fails, the car stores a short code and turns on the check-engine light. That is all you see: a light on the dashboard. The name of the problem is already inside the car. A shop often charges for the first hour just to plug in a scanner and read that name. WhyLamp shows the name in everyday words: what it means, how soon to act, what you can check yourself, and what we will not guess. Every fact has a link to where we found it. Then you decide if you need a shop, and what to ask.
What a reading includes
The name. The urgency. The limits.
01
The name
What the code means in everyday words, and which part of the car it is about.
02
The urgency
Whether you can drive a short way, should get it soon, or should stop if the light is flashing.
03
The limits
What you can check at home, what a shop will likely look at, and what we will not guess.
What you get
P0420
A real example: the name, how urgent it is, and links to our sources. No “usually it’s the converter.”
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
The car compared sensors before and after the part that cleans the exhaust. It decided that part is not working well enough.
Plans
Look up the code now. A plug-in tool can come later.
Looking up a code is free. You can do it now. Garage saves this car’s codes in one place. The Reader kit adds a small tool that plugs in under the dashboard. Garage and the kit are not for sale yet. We do not invent a price here.
Garage and the Reader kit are not for sale yet. There is no checkout and no invented monthly price. You can look up a code now.
Same name, or the brand’s own
Some codes mean the same thing on most cars. Some belong to the brand.
Many codes share a public name. That is why a cheap scanner can print P0420 on almost any car from 1996 on. Some codes still belong to Honda, Ford, or another brand.
Most cars
Shared
The same name on a Civic or an F-150. A shop may still check different things next.
One make
Brand-owned
The carmaker owns the meaning. The same five characters can name different problems. We will not guess.
Year and model
This car
Year and model still matter. Two cars can share a name and need different checks.
Trust
If we cannot show where a fact came from, we leave it off the page.
We show our sources
Code names come from a public list used across the industry. A note about your Honda or Ford needs its own link.
No invented repair
If the source page does not name a part, we leave it out. We will not guess a labor hour or a price.
Shared names vs the brand
Some codes share a public name on most cars from 1996 on. Some codes belong to the brand. We will not guess the second kind.
No one pays to rank here
We do not wear a car-brand badge. We do not take money from shops or parts stores to rank them. Looking up a code is free.
Sources: SAE J2012 — Diagnostic Trouble Code Definitions · U.S. EPA — Frequently asked questions about on-board diagnostics.
Questions