What is a diagnostic trouble code (DTC)?

A DTC is a standardized code your vehicle's computers store when they detect a fault — the reason your check engine light comes on. The first letter shows the system (P = powertrain, C = chassis/ABS, B = body, U = network communication), and the following digits narrow it down. A code is a pointer at a circuit or system that is out of range — not a diagnosis of which part failed.

Can I drive with a diagnostic trouble code?

A steady light with a minor code usually means you can drive to a shop. A flashing check engine light means active misfire that can destroy your catalytic converter in minutes — stop driving. Codes for charging voltage, lost module communication, or transmission limp mode should be diagnosed before putting meaningful miles on the vehicle.

Why do code readers give different repair quotes for the same code?

Because a code identifies a system reading out of range, and several different failed components can cause the same reading. Free code readers and auto-parts-store scans show the code and its most common cause — they do not test which cause applies to your car. Confirming the fault requires live data, waveform testing, and component-level checks, which is what a real diagnostic does.

How much does a professional diagnostic cost in Canyon Lake and the Hill Country?

Glass Box Diagnostics charges a flat $150 for a full mobile diagnostic (90 minutes, all modules scanned, live data analyzed, written report with root cause and repair path), with a $95 second-opinion review of another shop's diagnosis. The diagnostic fee is credited toward the repair if you have us do the work.

OBD-II Code Decoder — the honest version

Every other decoder tells you what the code means. This one tells you what it proves, what a parts-changer will try to sell you on it, and the test that settles it.

Codes we cover

Manufacturer-specific subcodes need a full-system scanner — the tools in my trunk read them all. Click any code to decode it.

Code	Name	System	Severity
P0420	Catalyst Efficiency Below Threshold (Bank 1)	Emissions / Engine	moderate
P0171	System Too Lean (Bank 1)	Fuel & Ignition	moderate
P0300	Random / Multiple Cylinder Misfire	Fuel & Ignition	serious
P0301	Cylinder 1 Misfire Detected	Fuel & Ignition	serious
P0442	Evaporative Emission System Leak (Small)	Emissions / Engine	minor
P0455	Evaporative Emission System Leak (Large)	Emissions / Engine	minor
P0128	Coolant Thermostat (Below Regulating Temperature)	Emissions / Engine	minor
P0335	Crankshaft Position Sensor Circuit	Electrical & Network	serious
P0562	System Voltage Low	Electrical & Network	moderate
P0700	Transmission Control System (MIL Request)	Transmission	serious
P0720	Output Shaft Speed Sensor Circuit	Transmission	serious
C0035	Left Front Wheel Speed Sensor Circuit	ABS / Safety	moderate
U0100	Lost Communication With ECM/PCM	Electrical & Network	serious
U0121	Lost Communication With ABS Module	Electrical & Network	moderate
B1234	Body / Restraint Circuit (Example B-Code)	ABS / Safety	minor
P0011	Intake Cam Timing Over-Advanced (Bank 1)	Emissions / Engine	moderate
P0340	Camshaft Position Sensor Circuit	Electrical & Network	moderate
P0401	EGR Flow Insufficient	Emissions / Engine	moderate
P0456	Evaporative Emission System – Very Small Leak	Emissions / Engine	minor
P0016	Crankshaft / Camshaft Correlation (Bank 1)	Emissions / Engine	serious
P0402	EGR Flow Excessive / Valve Stuck Open	Emissions / Engine	moderate
P0051	HO2S Heater Control Circuit Low (Bank 1 Sensor 2)	Fuel & Ignition	minor
P0073	Ambient Air Temperature Sensor Circuit High	Electrical & Network	minor
P0507	Idle Air Control System RPM Higher Than Expected	Fuel & Ignition	minor
U0073	Control Module Communication Bus Off	Electrical & Network	serious

Every other decoder tells you what the code means. This one tells you what it proves, what a parts-changer will try to sell you on it, and the test that settles it.

Codes we cover

Manufacturer-specific subcodes need a full-system scanner — the tools in my trunk read them all. Click any code to decode it.

Code	Name	System	Severity
P0420	Catalyst Efficiency Below Threshold (Bank 1)	Emissions / Engine	moderate
P0171	System Too Lean (Bank 1)	Fuel & Ignition	moderate
P0300	Random / Multiple Cylinder Misfire	Fuel & Ignition	serious
P0301	Cylinder 1 Misfire Detected	Fuel & Ignition	serious
P0442	Evaporative Emission System Leak (Small)	Emissions / Engine	minor
P0455	Evaporative Emission System Leak (Large)	Emissions / Engine	minor
P0128	Coolant Thermostat (Below Regulating Temperature)	Emissions / Engine	minor
P0335	Crankshaft Position Sensor Circuit	Electrical & Network	serious
P0562	System Voltage Low	Electrical & Network	moderate
P0700	Transmission Control System (MIL Request)	Transmission	serious
P0720	Output Shaft Speed Sensor Circuit	Transmission	serious
C0035	Left Front Wheel Speed Sensor Circuit	ABS / Safety	moderate
U0100	Lost Communication With ECM/PCM	Electrical & Network	serious
U0121	Lost Communication With ABS Module	Electrical & Network	moderate
B1234	Body / Restraint Circuit (Example B-Code)	ABS / Safety	minor
P0011	Intake Cam Timing Over-Advanced (Bank 1)	Emissions / Engine	moderate
P0340	Camshaft Position Sensor Circuit	Electrical & Network	moderate
P0401	EGR Flow Insufficient	Emissions / Engine	moderate
P0456	Evaporative Emission System – Very Small Leak	Emissions / Engine	minor
P0016	Crankshaft / Camshaft Correlation (Bank 1)	Emissions / Engine	serious
P0402	EGR Flow Excessive / Valve Stuck Open	Emissions / Engine	moderate
P0051	HO2S Heater Control Circuit Low (Bank 1 Sensor 2)	Fuel & Ignition	minor
P0073	Ambient Air Temperature Sensor Circuit High	Electrical & Network	minor
P0507	Idle Air Control System RPM Higher Than Expected	Fuel & Ignition	minor
U0073	Control Module Communication Bus Off	Electrical & Network	serious