車はコードで話す。
ここで意味が分かる。

OBD-IIコードや警告灯の意味、考えられる原因、安全にできる確認、運転を中止すべき状況を説明します。VINと公式リコールは希望した時だけ照会します。

OBD-II / LIVEA clear clue before replacing a part

警告を読み解く整備工場の診断に代わるものではありません。理解して判断するための手助けです。

AUTO / DIAG 01

何を確認しますか？

P0420

Guidance, not diagnosis

Catalyst system efficiency below threshold

Priority medium

The control module compares signals before and after the catalyst and sees less exhaust cleaning than expected. This code alone does not prove the catalyst must be replaced.

Possible causes

01Exhaust leak
02Aged or unstable oxygen sensor
03Degraded catalyst
04Incorrect mixture or combustion

Safe checks

Check related mixture or misfire codes.
Inspect for leaks only when the exhaust is cold.
Compare sensor data before replacing parts.

A code is a clue; it does not prove that one particular part has failed.

STEP 01

Read the warning

Enter the reader code, dashboard light or vehicle data.

STEP 02

Understand the scope

Separate meaning, possible causes and safety signals without false certainty.

STEP 03

Decide with context

Save or print a clear sheet before speaking with a workshop or replacing parts.

For people, not control modules

More context before opening the toolbox.

Buying a used car?

Check the VIN and review general safety campaigns before deciding.

Did a warning light appear?

Tell a routine service need from a signal that requires a safe stop.

Speaking with a workshop?

Bring a printable sheet with the code, symptoms and useful questions.

Clean-workshop privacy

No account. No vehicle history. No stored VIN.

Local explanations run in your browser. VIN and recall requests go to NHTSA only when you ask. Advertising is prepared for the future but remains disabled in this draft.

Back to the tool

車は コードで話す。ここで意味が分かる。