How do you interpret a DTC?

Each DTC corresponds to a specific vehicle malfunction, such as a misfire in engine one. The following common DTC, P0128, would be read like this: 

• The first letter is P, which means that the issue lies in the powertrain.
• The second character is 0, indicating that the following code is not manufacturer-specific code. It is standardized and can be interpreted according to SAE definitions.
• The third character is 1, which reveals that the issue is specifically within the fuel and air metering subsystem.
• The last two digits make up the specific fault index 28. 

Based on this reading, it’s clear that code P0128 refers to an issue of the engine coolant temperature that’s below thermostat regulating temperature.