Car Diagnostics — Warning Lights, OBD2 Error Codes & Solutions
Understand every dashboard warning light and OBD2 error code. Complete guide to diagnose your vehicle and avoid unnecessary repairs.
What is OBD2 Diagnostics?
OBD2 (On-Board Diagnostics) is a standardized system present in all vehicles sold in Europe since 2001 (petrol) and 2004 (diesel). It continuously monitors emissions and critical engine components.
When a problem is detected, the system turns on a dashboard warning light and records a DTC (Diagnostic Trouble Code). With a diagnostic scanner or OBD2 dongle, you can read these codes and identify the exact problem.
DTC Code Structure
P 0 4 2 0
P
= Powertrain (engine)
0
= Generic code (SAE)
4
= Auxiliary emission controls
20
= Specific fault
Dashboard Warning Lights — What They Mean
Each dashboard warning light indicates a specific problem. Red lights require immediate action, orange lights need a prompt diagnostic.
Engine Light (MIL)
Emissions or engine performance issue. From a loose gas cap to a catalytic converter problem.
Warning — Diagnostic needed
Oil Pressure
Oil pressure too low. Risk of engine damage if you continue driving.
Urgent — Stop driving
Battery / Alternator
Charging problem. The alternator is no longer charging the battery. Risk of imminent breakdown.
Urgent — Stop driving
Braking
Low brake fluid level or worn pads. Check immediately.
Urgent — Stop driving
ABS
Anti-lock braking system disabled. Normal braking works but without electronic assistance.
Warning — Diagnostic needed
Engine Temperature
Engine overheating. Stop immediately to prevent engine damage.
Urgent — Stop driving
Airbag / SRS
Airbag system fault. Airbags may not deploy in case of accident.
Warning — Diagnostic needed
Diesel Particulate Filter (DPF)
DPF clogged. Needs regeneration (drive on highway for 20 min) or cleaning.
Warning — Diagnostic needed
Most Common OBD2 Error Codes
Here are the DTC codes you'll encounter most often. Each code starts with a letter (P = Powertrain, B = Body, C = Chassis, U = Network) followed by 4 digits.
| Code | Description | Common Cause | Severity |
|---|---|---|---|
P0420 | Catalyst system efficiency below threshold | Worn catalytic converter, faulty O2 sensor | Medium |
P0300 | Random/multiple cylinder misfire detected | Worn spark plugs, ignition coils, injectors | High |
P0171 | System too lean (bank 1) | Vacuum leak, dirty MAF sensor, clogged injector | Medium |
P0442 | EVAP system leak (small) | Loose gas cap, cracked EVAP hose | Low |
P0301 | Cylinder 1 misfire detected | Cylinder 1 spark plug, coil, low compression | High |
P0128 | Coolant temperature below thermostat regulating temperature | Thermostat stuck open, temperature sensor | Low |
P0455 | EVAP system leak (large) | Missing gas cap, disconnected EVAP hose | Low |
P0401 | EGR flow insufficient | Clogged EGR valve, blocked EGR passages | Medium |
How to Diagnose Your Vehicle
Follow these 4 steps to identify and understand a problem detected on your vehicle.
1. Locate the OBD2 port
The OBD port is usually located under the dashboard on the driver side. On some vehicles, it may be behind a cover or near the fuse box.
2. Plug in the OBD2 scanner
Connect your diagnostic scanner or Bluetooth/WiFi OBD2 dongle to the port. Turn on the ignition without starting the engine. The device powers on automatically.
3. Read the error codes
Start the DTC code scan via the scanner screen or mobile app. Note all displayed codes — there may be several.
4. Interpret with the VIN
Enter your VIN in the Automano decoder to contextualize error codes against the exact model, manufacturer recalls, and known issues for your vehicle.
FAQ — Car Diagnostics
What is an OBD2 diagnostic?
OBD2 (On-Board Diagnostics) is a standardized system present in all vehicles since 2001 in Europe. It monitors emissions and engine components, generating error codes (DTCs) when a problem is detected.
How do I read my car's error codes?
You can use an OBD2 scanner plugged into the OBD port (usually under the dashboard on the driver side) or a Bluetooth/WiFi dongle with a mobile app. Automano then helps you interpret the codes.
What does the orange engine light mean?
The orange engine light (MIL) signals an emissions or engine performance issue. It can range from a loose gas cap to a catalytic converter problem. An OBD2 scan identifies the exact error code.
How much does a car diagnostic cost?
A basic garage diagnostic costs between €30 and €80. With a personal OBD2 scanner (€15-50), you can read codes yourself. Automano helps you interpret results for free.
Does OBD2 diagnostics work on all vehicles?
OBD2 is mandatory on all petrol cars since 2001 and diesel since 2004 in Europe. Older vehicles use proprietary protocols that require manufacturer-specific tools.
Can I drive with the engine light on?
A steady orange light generally allows careful driving to a garage. A flashing red light indicates a serious problem — stop immediately. In all cases, get a diagnostic quickly to avoid more costly damage.
Related Tools
Diagnostics are just the first step. Use the VIN to go further.
Free VIN Decoder
Decode the VIN to identify the exact model and access full history.
Free Valuation
Estimate the diagnostic impact on your vehicle's value.
Check Used Car
Always request a diagnostic before buying. Our guide explains why.
Sources: OBD2 codes per SAE J2012 / ISO 15031 · NHTSA vPIC data (National Highway Traffic Safety Administration)
Identify your vehicle by VIN
The VIN contextualizes each error code against the exact model. Manufacturer recalls, known issues, complete history.