DTCs and MIL Status
Purpose
To guide you on DTC and MIL status while using an Innova scan tool.
DTCs and MIL Status
When the vehicle’s onboard computer detects a failure in an emissions-related component or system, the computer’s internal diagnostic program assigns a diagnostic trouble code (DTC) that points to the system (and subsystem) where the fault was found. The diagnostic program saves the code in the computer’s memory. It records a 'Freeze Frame' of conditions present at the time when the fault was found and lights the Malfunction Indicator Lamp (MIL). Some faults require detection for two trips in a row before the MIL is turned on.
Note: The 'Malfunction Indicator Lamp' (MIL) is the accepted term used to describe the lamp on the dashboard that lights to warn the driver that an emissions-related fault has been found. Some manufacturers may still call this lamp a 'Check Engine' or 'Service Engine Soon' light.
There are two types of DTCs used for emissions-related faults: Type 'A' and Type 'B'. Type 'A' codes are 'One-Trip' codes; Type 'B' DTCs are usually Two-Trip DTCs.
When a Type 'A' DTC is found on the First Trip, the following events take place:
- When the failure is first found, the computer commands the MIL 'On'.
- If the failure causes a severe misfire that may cause damage to the catalytic converter, the MIL 'flashes' once per second. The MIL continues to flash as long as the condition exists. If the condition that caused the MIL to flash is no longer present, the MIL will light 'Steady' On.
- A DTC is saved in the computer’s memory for later retrieval.
- A 'Freeze Frame' of the conditions present in the engine or emissions system when the MIL was ordered 'On' is saved in the computer’s memory for later retrieval. This information shows the fuel system status (closed loop or open loop), engine load, coolant temperature, fuel trim value, MAP vacuum, engine RPM, and DTC priority.
When a Type 'B' DTC is found on the First Trip, the following events take place:
- The computer sets a Pending DTC, but the MIL is not ordered ON. 'Freeze Frame' data may or may not be saved at this time depending on the manufacturer. The Pending DTC is saved in the computer’s memory for later retrieval.
- If the failure is found on the second consecutive trip, the MIL is ordered ON. 'Freeze Frame' data is saved in the computer’s memory.
- If the failure is not found on the second Trip, the Pending DTC is erased from the computer’s memory.
The MIL will stay lit for both Type 'A' and Type 'B' codes until one of the following conditions occur:
- If the conditions that caused the MIL to light are no longer present for the next three trips in a row, the computer automatically turns the MIL OFF if no other emissions-related faults are present. However, the DTCs remain in the computer’s memory as a history code for 40 warm-up cycles (80 warm-up cycles for fuel and misfire faults). The DTCs are automatically erased if the fault that caused them to be set is not detected again during that period.
- Misfire and fuel system faults require three trips with 'similar conditions' before the MIL is turned OFF. These are trips where the engine load, RPM, and temperature are similar to the conditions present when the fault was first found.
Note: The 'Malfunction Indicator Lamp' (MIL) is the accepted term used to describe the lamp on the dashboard that lights to warn the driver that an emissions-related fault has been found. Some manufacturers may still call this lamp a 'Check Engine' or 'Service Engine Soon' light.
- Erasing the DTCs from the computer’s memory can also turn off the MIL. See Erasing Diagnostic Trouble Codes (DTCs), before erasing codes from the computer’s memory. If a Diagnostic Tool or Scan Tool is used to erase the codes, Freeze Frame data will also be erased.