OBD2 Code Decoder

Use when a check-engine light is on, an OBD2 scan returns P0xxx/P2xxx/B/C/U codes, or you need to know if the car is safe to drive, what the code means in plain language, likely causes with fix cost bands, and DIY difficulty. Decodes 70+ generic codes, detects multi-code patterns (lean-both-banks, misfire+catalyst chains), and gives a STOP-DRIVING / SOON / MONITOR verdict per code.

Overview

A check-engine light turns a $15 Bluetooth OBD2 dongle into a panic machine: you get "P0420" and Google gives you 40 possible causes from "loose gas cap" to "$2,400 catalytic converter". scripts/obd2_decoder.py converts codes into plain-language meaning, a safety verdict (STOP-DRIVING / FIX-SOON / MONITOR), ranked likely causes with realistic fix cost bands, DIY difficulty, and — the part Google never gives you — multi-code pattern detection that collapses several scary codes into one root cause.

When to Use

• Check-engine light is on (steady or flashing) and you have scanned codes.
• Buying/selling a used car and want to interpret the scan report.
• A code returned after a repair — is it the same fault or a new one?
• Deciding: drive to work tomorrow, drive to the shop, or tow it?

Don't use for: airbag/SRS crash data (B-code module dumps need manufacturer tools), hybrid/EV high-voltage faults, or anything involving the flashing light + burning smell + gauge-in-red combination — that is a shut-down-now situation regardless of codes.

How It Works

1. Decode each code against a 70+ entry generic-Powertrain database: plain-language meaning, system (fuel/air/ignition/emissions/sensor/ transmission), severity.
2. Pattern engine: known code COMBINATIONS that have a single root cause —
   • P0171 + P0174 (both banks lean) → vacuum leak / dirty MAF, not two O2 sensors
   • P0300..P0308 with P0171/P0174 → lean misfire; fix the lean condition
   • Misfire codes + P0420/P0430 → unburned fuel killed the cat; fix misfire FIRST or the new cat dies too
   • P0440-family together → evaporative system (often just a loose gas cap)
3. Verdict per code and for the whole set: worst severity wins, with the one-line reason. Flashing-CEI rule is always stated: flashing = active misfire dumping raw fuel = park it.
4. Cost bands & DIY per typical fix: e.g. P0420 — check exhaust leaks and rear O2 sensor ($20–150) before condemning the cat ($300–2,400); P0455 — try the gas cap ($0) before a smoke test ($80–150).
5. Clearing rules: when clearing is safe (after fixing), when it's illegal (emissions test fraud / masking an unfixed fault), and the drive-cycle note (monitors reset; codes often return within days if unfixed).

Using the Script

bash

# Decode a set of codes (the realistic case)
python3 scripts/obd2_decoder.py P0171 P0174

# Single code, full detail
python3 scripts/obd2_decoder.py P0420

# Verdict-only quick check
python3 scripts/obd2_decoder.py --short P0300

# List codes in one system, e.g. all evap codes we know
python3 scripts/obd2_decoder.py --list-system evap

Worked Example

Input P0171 P0174 P0300 outputs: both-banks-lean pattern detected → most likely vacuum leak or dirty MAF sensor (one cause, three codes), fix-first order = lean condition before plugs/coils, verdict FIX-SOON (safe to drive gently; long-term risk is catalytic converter damage), cost bands $0–20 (MAF clean), $20–80 (vacuum hoses), $150+ (intake gasket).

Common Pitfalls

1. Replacing the sensor the code names. O2 sensor codes are usually the sensor REPORTING a real mixture problem, not a broken sensor.
2. Clearing codes to pass inspection. Readiness monitors reset to "incomplete"; most inspection regimes reject a car with incomplete monitors, and the underlying fault returns.
3. Treating P0420 as an automatic cat replacement. Exhaust leaks, rear O2 sensors, and misfires all mimic it — the cheap causes first.
4. One code = one problem. The pattern engine exists because three codes are often ONE $15 vacuum hose.
5. Driving on a flashing CEL. Steady = diagnose soon; flashing = active misfire destroying the catalyst — stop driving.
6. Pending vs confirmed. A pending code (seen once) is information; a confirmed/stored code with the light on is a decision.

Verification Checklist

• Every input code got a decode or an explicit "manufacturer-specific — look up for your make" fallback, never silence.
• Pattern matches are shown BEFORE individual codes (root cause first).
• Whole-set verdict = worst severity among codes, with reason.
• Cost bands are ranges, never single promises.
• Clearing-rules block present when any emissions-related code appears.