FAQ – What is a DTC?

DTC stands for OBDII Diagnostic Trouble Code, and it’s the single best way for your to diagnose problems with your car or truck. With OBD II, different DTC faults have different
priorities according to regulations. As a result, the priorities determine MIL illumination and trouble code erasure. DTCs are entered according to individual priority. DTCs with a higher priority overwrite a lower priority DTC.

DTC priorities are dictated in several different ways, these priorities are set by the manufacturer to inform you of the severity of the problem. These rank the problem with your car, alerting to if there’s a more serious issue at hand that needs to be taken care of.


DTC Priorities

There are 4 distinct levels of OBDII trouble code, each of which can impact your car or truck differently. Priority 0 is for Non-emissions related trouble codes, while Priority 1 DTC trouble codes refer to one trip errors or failures during a two trip fault. These can also range from ignition misfire issues, or for non-fuel system and non-misfire. Priority 2 DTC trouble codes can vary, from one trip failure of a two trip fault as well as fuel system errors for your engine being too rich or too lean. These trouble codes can also qualify for misfire problems or intermittent misfire as well.

Starting from priority 3 DTC trouble codes, your car can go into limp mode if the error is hard enough. These DTC codes range from two trip failures for a non-fuel system and non-misfire or matured one trip comprehensive component fault.

The final priority DTC is Level 4 and these errors are two trip failures or a matured fault for fuel system. These can definitely trigger limp mode as your engine runs too lean, or too rich, and can include many of the oxygen sensor trouble codes. Of course extreme cases of misfire and detonation are sure fire priority 4 DTC trouble codes, and almost guaranteed to trigger limp mode.

Non Priority DTC Trouble Codes

Any non-emissions related diagnostic trouble codes or failures will qualify as having no priority. Because these are typically one trip failures of two trip faults low priority almost never causes a limp mode. Two trip failures or matured faults have higher priority and usually qualify as one of the DTC codes above.. One and two trip failures of fuel system and
misfire monitor take precedence over non-fuel system and non-misfire failures.

DTC Self Erasure

With one trip components or systems, the MIL is illuminated upon test failure and trouble codes are stored. Two trip monitors are components requiring failure in two consecutive trips for MIL illumination. Upon failure of the first test, the Task Manager enters a maturing code. If the component fails the test for a second time the code matures and a DTC is set.

After three good trips the MIL is extinguished and the Task Manager automatically switches the trip counter to a warm-up cycle counter. OBDII check engine trouble codes are automatically erased following 40 warm-up cycles if the component does not fail again. For misfire and fuel system monitors, the component must pass the test under a Similar Conditions Window in order to record a good trip. A Similar Conditions Window is when engine RPM is within 375 RPM and load is within 10% of when the fault occurred.

NOTE: It is important to understand that a component does not have to fail under a similar window of operation to mature. It must pass the test under a Similar Conditions Window when it failed to record a Good Trip for trouble code erasure for misfire and fuel system monitors.

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