For a 0.020 inch leak, the DTC(s) shall be a generic SAE J2012 DTC that is specific to an EVAP system very small leak (e.g., P0456, P04EE, or P04EF) or an equivalent manufacturer-specific DTC that we approve.Checked 2026-08-12
Diagnostic Trouble Code (DTC)
The code an OBD-II system stores to identify which monitored system it found a fault in.
Also called: DTC · fault code · engine code
What it means
Definition
A DTC is the standardised code the vehicle's computer stores when a monitored system reports a fault. Federal emissions regulation refers to these as generic SAE J2012 codes and cites examples in the P0xxx form. Some emissions codes are permanent: the Bureau of Automotive Repair notes that permanent DTCs are the same as regular codes but cannot be erased with a scan tool or by disconnecting the battery.
Why a buyer cares
What it changes
A stored code names the system the computer flagged, so a scan before purchase gives a starting point independent of what the dashboard is showing at that moment.
In practice
Where you meet it
On a scan-tool printout, an inspection report, or a mechanic's diagnostic summary, written like P0456.
Two things that look alike
Not the same as
The code is stored data; the dashboard lamp is what the system switches on once a code is confirmed.
Read next
Related terms
OBD-II (On-Board Diagnostics II)
The standardised self-diagnostic system in modern cars, read through a connector under the dash.
Malfunction Indicator Lamp (MIL)
The dashboard lamp — usually labelled Check Engine — that the OBD-II system switches on when it confirms a fault.
Readiness monitors
Self-tests the OBD-II system must complete before an emissions test can be read.
Where this comes from
Sources
PDTCs are the same as regular diagnostic trouble codes (DTC), but they cannot be erased by clearing the DTCs with a scan tool or by disconnecting the vehicle's battery.Checked 2026-08-12
What the letter prefixes P, B, C and U stand for was not sourced, so the entry leaves it out. No claim is made about what any individual code number means beyond the regulation's own EVAP example.