Q. Why Perform OBD II Inspection?

A: On-Board Diagnostic systems (OBD) on 1996 and newer vehicles will be checked as part of Delaware’s vehicle inspection program. OBD technology benefits motorists, repair technicians and our environment. It’s good for motorists because it monitors the vehicles performance every time it is driven and identifies problems immediately allowing service to be done before more serious problems develop. It’s good for repair technicians because it helps to accurately diagnose problems, allowing for efficient and proper repairs. And it’s good for the environment and our health because it identifies problems that cause vehicle emissions to increase.


  1. The Malfunction Indicator Light (MIL) does not illuminate at all when the ignition key is turned to the “key on, engine not running” position. Depending on the vehicle make, the MIL will display something similar to “Service Engine Soon” or “Check Engine”.
  2. The MIL status, as indicated by the scan tool, is ON. The purpose of inspecting the MIL status is to determine if the vehicle’s OBDII system has commanded the MIL to turn on based on a malfunction. In most cases, the MIL should be illuminated when the engine is running and there is a malfunction.
  3. More than the allowable numbers of monitors are not ready. The purpose of the readiness status check is to determine if the vehicle’s OBDII system has tested all emission control components. If the vehicle’s battery has been recently disconnected, or if trouble codes have been recently cleared with a scan tool, OBD components will be set to “not ready”.

Q. What is OBD II?




Q. How is the driver notified?

A: By lighting a special lamp in the dash called a Malfunction Indicator Lamp (MIL). This light is reserved for actual malfunctions that can cause increased emissions, and cannot be used for other purposes such as a reminder for regularly scheduled maintenance. The MIL is turned on when the vehicle’s OBD-II system finds a problem that can affect emissions. Depending on the vehicle make, the MIL will display something similar to “Service Engine Soon ” or “Check Engine”.

Q. Which light is the OBD-II Malfunction Indicator Lamp (MIL)?

A: Automobiles use a variety of warning lamps to notify drivers of different conditions. However, in the case of the MIL, either the phrase “Service Engine Soon” or “Check Engine,” or a standard symbol such as the one below is used.

Q. What does it mean if the MIL is on?

A: OBD II turns the MIL on when a failure occurs which could cause vehicle emissions to exceed 1.5 times their designed standard. The MIL also illuminates when a problem is detected in a component that is used as part of the diagnostic strategy for any other monitored system or component. The purpose of the MIL is to alert the driver to the malfunction so service can be performed in a timely manner.

If the MIL is flashing, vehicle operation is discouraged until the malfunction is repaired.

Refer to your vehicle owner’s manual for specific information about your vehicle’s MIL.

Q. What does the Delaware's inspection system check?

A: There are two basic steps to the OBD II inspection:

  1. With the key on and engine off, the MIL bulb is checked to verify that it works, and:
  2. The inspection tool is connected to the on-board computer that checks the OBD-II system status.

The following is a typical OBD-II connector located in your vehicle.

Three pieces of information are downloaded from the vehicle’s OBDII system:

  1. Emissions MIL command – on or off;
  2. Which, if any, fault codes are stored; and
  3. Readiness status of OBD II system.

Q. What are fault codes?

A: Before OBD-II, each manufacturer had a specific trouble code list and code definitions. Under the OBD-II requirements, all manufacturers must comply with a standardized convention for Diagnostic Trouble Codes (DTCs). These fault codes are stored in the vehicle’s computer to indicate which system or component might be causing the problem. These fault codes can help technicians to make effective and efficient repairs.

Q. What is readiness status?

A: To help ensure the OBD system is working properly, “readiness codes” are used to indicate whether or not monitored emissions control systems have been tested by the OBD system. Each emissions control system has its own monitor and related readiness code. If any of the readiness codes are set to "not ready" or "not complete," the OBD system has not yet completed testing of that particular component or system. A component failure may exist, but has not yet been identified because the system testing has not been completed.

Q. How do I pass the OBD II emission inspection?

A: There are two basic steps to the OBD test:

  1. With the key on and the engine off, the Malfunction Indicator Lamp (MIL) is checked to verify that it works; and,
  2. A cable is attached to the on-board computer connection (called a Data Link Connector or DLC) that checks the OBD system status.

Three pieces of information are downloaded from the vehicle:

  1. The emissions MIL command - on or off; and,
  2. Which fault codes are stored; if any, and,
  3. Readiness monitor status.

Essentially, three things must be satisfied to pass the test:

  1. The MIL must illuminate with the key on and engine off; and,
  2. The MIL must be commanded off by the OBD system; and,
  3. Readiness monitors must be completed.

You can easily verify that you will pass the 1st criteria by confirming that the MIL bulb lights-up when the key is turned to the full on position without starting the engine. Then start the engine and see if the MIL stays on. If it is on, you will fail the 2nd criteria, so your vehicle must be repaired before you can pass the test. You will likely pass the 3rd criteria unless your vehicle’s battery has been recently disconnected, or a technician has recently cleared fault codes.


Q. My car didn't pass. Now what should I do?

A:It depends on why it didn’t pass, but you will probably need to take your vehicle to a repair facility for service.

  • If the Malfunction Indicator Light (MIL) is on (the light that says “check engine” or “service engine soon”), the OBD II system has found a problem that needs to be fixed.
  • If the MIL won’t light, then the bulb and/or circuitry must be repaired.