What is OBD-II?

OBD-II (on-board diagnostics, second generation) is the self-check system built into the engine computer of modern cars and light trucks. It watches the parts that affect exhaust emissions. When it finds a fault, it turns on the check engine light and stores a trouble code. Any standard scan tool can read that code through a 16-pin socket under the dashboard.

Does my car have OBD-II?

In the United States, these vehicles have it:

• All 1996 and newer gasoline passenger cars and light trucks. Vehicles that can also run on an alternative fuel have full OBD-II when running on gasoline; full OBD-II while running on the alternative fuel itself was required from the 2005 model year.
• All 1997 and newer diesel passenger cars and light trucks.
• A small number of 1994 and 1995 gasoline vehicles.

To check a specific vehicle, look at the emission control information label under the hood. An OBD-II vehicle says "OBD II" on it.

In the European Union the same idea is called EOBD. Directive 98/69/EC required it on petrol passenger cars up to 2,500 kg for new models type-approved from 1 January 2000 and for all new cars from 1 January 2001; heavier petrol cars followed a year later (2001 and 2002). Diesel passenger cars up to 2,500 kg and six seats followed: new models from 1 January 2003, all new cars from 1 January 2004. Heavier diesel passenger cars needed it on new models from 1 January 2005. EOBD uses the same connector and the same code format.

What OBD-II watches

The computer runs tests, called monitors, on emission-related systems. On a gasoline car they include:

• Catalytic converter efficiency.
• Engine misfire.
• Fuel system (how far the computer has to correct the fuel mixture).
• Oxygen sensors and their heaters.
• Evaporative system (fuel vapor leaks).
• Exhaust gas recirculation (EGR) and secondary air, where fitted.
• Every electronic input and output of the engine computer that can affect emissions.

Newer vehicles add checks such as the cooling system, crankcase ventilation and variable valve timing. Diesels have their own list, including the particulate filter and NOx aftertreatment. See drive cycles and readiness monitors for the full list.

The check engine light

The official name is the malfunction indicator lamp (MIL). It usually reads "Check Engine" or "Service Engine Soon".

• Steady light: a fault has been confirmed. For most faults that happens when the same problem is seen on two driving cycles in a row.
• Blinking light: a misfire bad enough to damage the catalytic converter. Have the car serviced as soon as possible.
• Light goes out by itself: once the fault is not seen again on three driving cycles in a row.

The light is not only about emissions. Many faults that turn it on also hurt fuel economy, driveability or performance.

What a scan tool can read

Every OBD-II vehicle must give a generic scan tool the same basic information:

• Stored (confirmed) trouble codes and pending codes. Permanent codes were phased in from the 2010 model year and are required on all vehicles from 2012.
• A freeze frame: a snapshot of engine data from the moment a fault was stored.
• Live data, such as engine speed, coolant temperature, vehicle speed, calculated load, throttle position, fuel trim and oxygen sensor output.
• Readiness status: whether each monitor has finished its self-test since codes were last cleared.
• Whether the check engine light is commanded on.

To learn what a code like P0420 is made of, read how to read an OBD-II code. To look a code up, use the trouble code list.

What is standardized

OBD-II made five things the same on every vehicle:

Since the 2008 model year, US cars and light and medium-duty trucks (up to 14,000 lb gross vehicle weight rating) must use ISO 15765-4 (CAN). Heavy-duty trucks use SAE J1939. The connector and pinout guide shows which pins each protocol uses.

What OBD-II does not do

• It does not name the broken part. A code points to the likely area of the fault. A lean code like P0171, for example, can come from a vacuum leak, a weak fuel pump or a dirty air flow sensor.
• It does not cover the whole car. The rules cover emission-related systems. Codes from ABS, airbag and body modules are outside the required set, and reading them usually needs a tool that supports the manufacturer's own diagnostics.
• Clearing a code does not fix anything. Clearing erases stored codes and resets every monitor to "not complete". If the fault is still there, the code comes back.

OBD-II and emissions tests

Many inspection programs now test cars by plugging into the OBD-II socket instead of measuring the tailpipe. Under the US federal rule, a vehicle fails if the check engine light is commanded on with a stored code. It also fails if the light does not come on at all with the key on and the engine off. Too many incomplete monitors means the car is sent away to be driven and tested again. See drive cycles and emissions tests for the limits.

For the story of how OBD-II came about, see the history of OBD-II. For the terms used here, see the OBD-II glossary.

OBD DashX reads and clears OBD-II trouble codes on Android with a Bluetooth ELM327 adapter.