# Freeze Frame

> OBD-II trouble codes, diagnosed in the right order

Plain-language reference for OBD-II diagnostic trouble codes. Every code page states what the engine computer actually measured, ranks the likely causes by how often each turns out to be responsible, identifies the single measurement that separates a cheap repair from an expensive one, and orders the diagnostic steps cheapest and most likely first.

Content is generic (SAE-standard) code information, not vehicle-specific service data. Cost figures are US dollars and are typical ranges, not quotes.

## Codes

- [P0128 — Coolant Thermostat Below Regulating Temperature](https://www.freezeframecode.com/codes/p0128/): the engine is not getting hot enough, fast enough, and nine times out of ten the thermostat is stuck open.
- [P0171 — System Too Lean (Bank 1)](https://www.freezeframecode.com/codes/p0171/): the engine computer has run out of room adding fuel to correct a mixture that keeps reading lean on bank one.
- [P0300 — Random / Multiple Cylinder Misfire Detected](https://www.freezeframecode.com/codes/p0300/): more than one cylinder is failing to burn its charge properly, and the computer can't pin it to a single one.
- [P0401 — Exhaust Gas Recirculation Flow Insufficient](https://www.freezeframecode.com/codes/p0401/): the computer opened the EGR valve, looked for the effect that should follow, and didn't find it — usually because carbon has blocked the passages.
- [P0420 — Catalyst System Efficiency Below Threshold (Bank 1)](https://www.freezeframecode.com/codes/p0420/): the catalytic converter on bank one isn't cleaning the exhaust as well as the computer expects — or the sensor watching it is lying.
- [P0455 — Evaporative Emission System Leak Detected (Large Leak)](https://www.freezeframecode.com/codes/p0455/): the fuel vapour system failed a sealing test by a wide margin, and about half the time the cause is the fuel cap.

## Guides

- [How to read an OBD-II trouble code](https://www.freezeframecode.com/how-to-read-a-code/): Where the port is, what the five characters mean, and why the code is a starting point rather than an answer.
- [Choosing a scan tool that can actually diagnose](https://www.freezeframecode.com/choosing-a-scan-tool/): The one feature that matters, the three tiers of tool, and what you genuinely need for the diagnostics on this site.
- [About Freeze Frame](https://www.freezeframecode.com/about/): How these diagnostic guides are put together and what the ordering principle is.

## Reference

- [Glossary](https://www.freezeframecode.com/glossary/): definitions of the terms used in these procedures
- [Browse by system](https://www.freezeframecode.com/systems/): codes grouped by subsystem