DTC fault codes are easy way of letting you know that your engine has a problem. Every sensor and circuit on your engine has a certain expected reading and voltage signal.
DTC, as you can already surmise from the title, is an acronym for "Diagnostic Trouble Codes". Today's computerized engine control system can up to a certain point self-diagnose to detect auto problems that could be affecting your vehicle's emissions and engine performance.
This also goes for the onboard systems and the antilock brake systems controlled by your engine's computer.
First unit identifies the type of error code:
Uxxxx for class 2 network
Second digit shows whether the code is manufacturer unique or not:
x0xxx for government-required code
x1xxx for manufacturer-specific code
Third digit shows us what system the trouble code references:
xx1xx/xx2xx show air and fuel measurements
xx9xx notates input/output signals and controls
Digits four and five show the specific failure code.
xxx00 to xxx99 - these are based on the systems defined in the third digit.