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.
• P0430 — Catalyst efficiency, Bank 2 — and what both bank codes at once really means.
• P0442 — Small EVAP leak — the cap seal, the slow monitor, and why this one lingers.
• P0456 — Very small EVAP leak — the pinhole class, and the one code that really needs smoke.
• P0128 — Engine won't reach temperature — thermostat or sensor, and the warm-up curve that tells you.
• P0011 — Cam timing over-advanced — the code where the dipstick is the first diagnostic tool.
• P0102 — MAF signal low — a circuit code, not a cleaning code. Start at the connector.
• P0113 — Intake air temp circuit high — and the -40° reading that gives it away instantly.
• P0135 — O2 heater circuit — why the sensor usually gets replaced, and what to rule out first.
• P0174 — System too lean, Bank 2 — and why its twin showing up too changes everything.
• P0302 — Cylinder 2 misfire — and whether a shared ignition coil is the reason.
• P0303 — Cylinder 3 misfire — and how a second code on its partner cylinder changes the diagnosis.
• P0304 — Cylinder 4 misfire — and confirming which physical cylinder that is before anything comes apart.
• P0700 — Transmission control system malfunction — a signpost code, not a diagnosis.
• P0441 — Incorrect purge flow — a flow problem, not a leak. Different code, different repair.
• P0299 — Turbo underboost — the engine isn't getting the boost it asked for. Live data shows how much.
• P0014 — Exhaust cam over-advanced — the same oil-first check as P0011, on the other cam.
• P0101 — MAF range/performance — the signal is alive, it just doesn't add up. Different fault than P0102.
• P0172 — System too rich — the mirror of P0171, with a different cause list entirely.

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.
• Cleaning a MAF sensor — How to clean it properly — and how to tell first whether it's actually the problem.
• Vacuum leak symptoms — Rough idle that smooths out at speed — and the trim pattern that proves it.
• Car won't start — Rapid clicks, one clunk, or healthy cranking — the sound narrows it before any tool.
• Hesitation on acceleration — A stumble under load is fuel, air, or spark — captured data tells the three apart.
• How to test an alternator — Battery or alternator? Voltage tells you in minutes — with a meter or without one.
• Parasitic draw test — A battery dead again by morning? Here's the multimeter test that finds what's draining it.
• Bad spark plug symptoms — Rough idle, weak acceleration, worse mileage — and the free test that tells you it's the plugs.
• EVAP system leak — Fuel smell, a gas-cap message, or a failed emissions test — which code you actually have, and where to start.

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.

In Fixiom the scan tool is free forever — read and clear codes, live data, freeze frames, emissions checks, the battery test, and the trip computer. Credits only cover the AI reasoning on top. 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