OBD-II fault code lookup

An online catalogue of OBD-II fault codes — P, B, C and U. Paste whatever the scanner showed: the reading covers each code separately and their combination together, because several codes nearly always share one cause and fixing them one at a time is the most expensive route. For each code the tool names the system, the likely causes in order of likelihood, and what you can check yourself before the garage. Codes that are not in the list still get an answer: five characters by themselves say which system the fault belongs to and whether the code is a generic one or specific to the make.

How to use the catalogue

Paste everything the scanner showed

Not one code at a time but the whole thing: the codes are found inside the text even with descriptions and stray lines from the app around them.

Read the combination first

If there are several codes, the block at the top says what they mean TOGETHER. That is often one cause instead of four separate repairs.

Start from the top of the causes

The list is sorted by likelihood. The first entries are cheaper to check and are more often the culprit.

Do the checks before the garage

Retighten the fuel cap, look at the oil level, swap a coil — sometimes that is enough, and if it is not, you arrive with a narrowed field.

Find out what the light on the dash means before you book a garage

Codes from the scanner

You can paste the whole list from the app: the codes are found inside the text. Ctrl+Enter to look up.

✉️ Know a code we do not have?

Send it in — we go through them by hand and add them to the catalogue. Manufacturer codes are the most valuable: they are outside the generic standard and are nearly impossible to find online.

The submission goes into a queue for manual review. We do not reply by email — the reading simply appears in the catalogue.

What the catalogue does

Reads the COMBINATION, not just each code alone

P0171 on its own is almost always an air leak near its own bank. P0171 together with P0174 is a cause shared by both banks: the intake, the mass air flow sensor or fuel pressure. Catalogues answer about a code; people arrive with a set.

Answers even for a code that is not in the list

Five characters carry structure: the letter is the system, the second digit says generic or manufacturer-specific, the third is the subsystem. So instead of «no reading found» the tool says what the fault belongs to and why a universal reading cannot exist for it.

Causes ordered by likelihood, not by price

The list starts with what is most often at fault and cheapest to check. For P0420 that is the rear oxygen sensor and an exhaust leak, not the catalyst the code names in its title.

Says whether you can keep driving

Urgency differs per code: an evaporative leak can wait for the next service, while overheating means pulling over as soon as it is safe. Those are different answers, and a catalogue that gives the same one is useless.

Suggests checks you can do yourself

Swap a coil with a neighbouring cylinder, retighten the fuel cap, look at fuel trims in the same scanner — short steps that often settle the question before the garage, or at least narrow the search.

Grows from real submissions

Manufacturer codes are outside the generic standard: the same characters mean different things on different makes. You can send one in through the form — submissions are reviewed by hand and added to the catalogue.

Why a code is not a diagnosis

A fault code says what the control module NOTICED, not what broke. «Catalyst efficiency below threshold» literally means «the rear oxygen sensor reads almost the same as the front one» — and that can be a dead catalyst, a worn sensor or a crack in the exhaust. A catalogue that translates the code title into plain words and stops there helps very little: it only repeats what the scanner screen already said. What helps is a reading that names what the module actually measured, which causes produce that measurement, and in what order they are cheapest to check.

Questions about fault codes

The scanner showed four codes. Which one do I start with?

Not the first on the list and not the scariest by name, but their combination. Several codes nearly always share one cause: a sagging voltage throws a batch of network codes, an air leak gives a lean mixture on both banks and then a catalyst code. That is why the «what the combination says» block sits above the individual readings here — if it fired, that is what needs fixing.

Does P0420 mean the catalytic converter has to be replaced?

Usually not. The module does not measure the catalyst directly: it compares the oxygen sensors before and after it. The code appears when the rear sensor starts reading almost the same as the front, and there are several reasons for that — a worn rear sensor, a small exhaust leak ahead of it, or misfires that really did burn the catalyst. Starting with the most expensive part is the most common and most expensive way to get it wrong.

What do the letter and the first digit mean?

The letter is the system: P is engine and transmission, B is body electronics, C is chassis, U is communication between modules. The second digit says whose code it is: 0 and 2 are the generic standard, identical on every car; 1 and 3 are manufacturer codes. That is exactly why manufacturer codes have no universal reading — the same characters mean different things on a Toyota, a VW and a Renault.

Can I just clear the code and keep driving?

You can, and the light will go out — but the fault stays, and so do its consequences. Worse, while an old code is present a new one can go unnoticed, and after clearing, the module has to run all its internal monitors again, which takes dozens of trips. For an emissions test that means the car will not pass for a while after clearing, even if everything is genuinely fixed.

Why does my code say the reading «depends on the make»?

Because its second digit is 1 or 3. The generic OBD-II standard only describes codes with 0 and 2; everything else manufacturers define for themselves, and those lists are far from always published. Such a code still gets a structural reading here — the system and subsystem are known — and you can send the exact meaning through the form if a garage named it.

Does the tool read codes straight from the car?

No, and that matters: reading codes needs an adapter in the diagnostic socket, and a browser has no access to it. What is read here is what you already got from a scanner or an app. You can paste it as it came — the codes are found inside the text.

💡 Want us to improve this tool just for you?

We can! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

How do you rate this tool?

Thank you for your rating!

Change my rating

Want to leave a comment?

Want to share more? Leave a comment!

Thank you! Your comment will appear after moderation.

Who is this tool for?

🚗 Drivers

Published 10 Sep 2026 Author: The Supreme