Code & symptom guides

Code guides

You have a specific code and want to know what it really means before anyone quotes you a part.

• P0420 — Catalyst efficiency below threshold — the code people get quoted a converter for.
• P0171 — System too lean — where fuel trims tell you the answer in about two minutes.
• P0300 — Random misfire — and the per-cylinder counters that show which one it really is.
• P0301 — Cylinder 1 misfire — and the swap test that proves the cause in one drive.
• P0455 — Large EVAP leak — check the gas cap first, and don't clear it before an inspection.

Symptom guides

Something's wrong and there's no code yet — or the light is on and you want to know how worried to be.

• Check engine light flashing — A flashing light is not the same warning as a steady one. What to do right now.
• Check engine light on — The triage page — how urgent is it, and what to do in what order.
• How to clear a check engine light — You can. Here's what it actually costs you, and when not to.
• Car shaking at idle — With a check engine light and without are two different diagnoses.

What these are for

Most code articles online are dictionary entries: here is the code, here is the official description, here are five parts you might buy. They're written to be found, not to be useful, and they leave out the part that actually saves money — how to tell which of those five it is.

These guides are built the other way round. Each one centres on a test you can genuinely run from the driver's seat with a cheap Bluetooth LE adapter: watch these two sensors, compare them under these conditions, and here's what each result rules out. That's how the problem gets narrowed before anything is taken apart.

Two rules run through all of them:

• A code narrows. Tests confirm. No code, on its own, identifies a failed part. Anything claiming otherwise is selling something.
• The limits get stated. Every guide ends with what a phone adapter genuinely cannot see — smoke testing, compression, backpressure, physical inspection. A tool that won't tell you where it stops isn't being confident, it's being useless.

Reading codes, watching live sensor data, checking emissions readiness, and testing the battery are free in Fixiom — always, no credits and no subscription. Every test described across these guides sits inside that free half.

Also useful: what an AI mechanic can honestly do, and the adapter buying guide covering which Bluetooth LE units actually work with third-party apps.

Common questions