Diagnostic Trouble Codes & Warning Lights: The Complete Workshop Guide
1.0 Introduction to On‑Board Diagnostics
Modern vehicles use standardized OBD‑II systems. When a fault is detected, the ECU stores a Diagnostic Trouble Code (DTC) and often illuminates a warning light. This guide covers all colour codes, common DTCs, and repair estimates. Use the quick calculator in section 5.0 to estimate costs.
2.0 Warning Light Colours & Meanings
2.1 Red – immediate stop
Red lights indicate critical safety issues (brake failure, overheating, low oil pressure). Stop the engine immediately.
2.2 Yellow/amber – service required
Yellow warns of a malfunction that needs attention soon (engine management, ABS, tyre pressure). You may continue but avoid high load.
2.3 Green/blue – information only
Lights like high beam, turn signals, or cruise control active. No action needed.
2.4 Common dashboard light table
| Symbol | Name | Colour | Typical DTC | Action |
|---|---|---|---|---|
| ⚠️ | Brake system | Red | C1011–C1015 | Check fluid, handbrake |
| 🔧 | Check engine | Yellow | P0420, P0300 | Scan for codes |
| 🛞 | TPMS | Yellow | C1240–C1250 | Check tyre pressure |
| 🌡️ | Coolant temp | Red | P0115–P0119 | Stop engine, check coolant |
| ⚡ | Battery charge | Red | P0560–P0563 | Check alternator belt |
3.0 Diagnostic Trouble Code (DTC) structure
DTCs consist of a letter and four digits: P (powertrain), C (chassis), B (body), U (network).
3.1 Most frequent codes
| Code | System | Description | Typical fix cost (£) |
|---|---|---|---|
| P0420 | Engine | Catalyst efficiency low (bank 1) | 250–800 |
| P0302 | Ignition | Cylinder 2 misfire | 80–300 |
| C0035 | ABS | Left front wheel speed sensor | 90–220 |
| B1000 | Body | ECU internal fault | 400–1200 |
4.0 Advanced diagnostics & live data
Professional scanners read live sensor data (fuel trim, O2 voltages, misfire counts). Always record freeze frame data before clearing codes.
5.0 DTC Repair Cost Calculator
Estimate your expense
*Rough guide – actual costs vary by vehicle and garage.
6.0 Frequently Asked Questions (FAQ)
If it’s solid yellow, you can usually drive a short distance, but get it diagnosed soon. If flashing, stop immediately (catalyst damage risk).
Catalyst efficiency below threshold. Often requires catalytic converter replacement, but sometimes an oxygen sensor or exhaust leak.
You can use an OBD‑II scanner to clear codes, but if the fault persists, the light will return. Always fix the root cause first.
Usually a wheel speed sensor failure (code C0035 etc.) or low brake fluid. Diagnostics needed.
7.0 Conclusion & recommendations
Always treat red lights urgently. Use a reliable OBD‑II scanner, and when in doubt consult a professional. Bookmark this guide for quick reference.
