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The OBD2 modes, one by one

When an app asks the car something over OBD, the first thing it sends is the mode number, which the standard calls a service. There are ten, and five of them do almost everything.

In short: when an app asks the car something over OBD, it first says what kind of question it is, the mode. Mode 01 is live data, 03 the stored faults, 04 clears them and 09 gives the VIN. With Final OBD you do not need to know which is which: it uses them all for you. Inside you have the detail, mode by mode, for anyone who wants to know what happens underneath.

Open the bonnetthe in-depth explanation
ModeWhat it asks forWhat it’s for
01Live dataRevs, speed, temperatures, oxygen sensors, fuel trims, the warning light and the monitors. Each value has its own number, the PID.
02Freeze frameHow the engine was running when the fault was stored: revs, load, temperature, speed.
03Confirmed fault codesThe ones that turn on the check engine light.
04ClearClears the fault codes and the freeze frame, turns off the light and resets the monitors.
05Oxygen sensor testsOnly on cars that don’t use CAN; on CAN it’s part of 06.
06On-board test resultsThe figures from each check with their limits: catalytic converter, misfires per cylinder, EGR.
07Pending fault codesFaults seen once that are waiting to be confirmed.
08Run a testFor example, the fuel vapour leak test. Few cars offer it.
09Vehicle informationThe VIN, the control unit’s software version and the monitor counters.
0APermanent fault codesMode 04 doesn’t clear them: they go away on their own once the car has checked they’re fixed.

01: live data

It’s what the gauges use. The app asks for a PID (0C is revs, 05 is engine temperature) and the car answers with one or more bytes that have to be converted using the formula in the standard. Each car only supports some of them, and the first thing an app does is ask which ones it has. What each one means and which value is normal is in the OBD2 PIDs.

03, 07 and 0A: the three fault lists

Mode 03 gives the confirmed faults, the ones that turn the light on. Mode 07, the pending ones: the car has seen the fault once and is waiting to see it again before confirming it. Mode 0A, the permanent ones, which exist on more recent cars and can’t be cleared by hand: they’re what the ITV looks at to catch a last minute clear. What each code means is in the list of fault codes.

04: clearing, with care

Mode 04 clears the faults, the freeze frame and the emissions monitors in one go, so the car will have to complete them again while driving. It doesn’t fix anything: if the cause is still there, the code comes back. It’s covered in how to read and clear fault codes and in the readiness monitors.

09: the VIN and the software

Mode 09 gives the vehicle identification number (VIN), the calibration IDs of the control unit’s software and how many times each monitor has completed. Older cars sometimes don’t give the VIN this way.

Beyond the standard modes

For ABS, airbag or body control units and manufacturer data, car makers also use specific diagnostic services, such as 22 to read data by identifier and 19 to read fault codes. Final OBD uses those services to read the faults in every control unit and your manufacturer’s own data: see what your car gets in the list of compatible cars.