How many control units does your car let you read? Engine, ABS, airbag, air conditioning
What the standard requires, and nothing more
OBD2 was born from an emissions requirement. The law requires the engine control unit (and the control units that affect emissions, such as an automatic gearbox's) to answer any tool, with commands and a code format that are identical for all makes: fault codes, live sensors, emissions checks, VIN. This is what the “Read fault codes” button reads.
The ABS, airbag, air conditioning, instrument cluster, body control module or doors are not covered by this requirement. They are connected to the diagnostic port, but each manufacturer chooses how they present themselves: network addresses, protocol, code format. Some answer standardised commands, others only the make's own tools.
How many control units in a car?
An older small city car has a few; a well-equipped recent car can have several dozen. In our tests, a Mazda CX-30 showed 31 and a BMW 2 Series about twenty. This number is not a quality score: it is only what the car lets you see at the port.
What decides how many can be read
- The car: some makes open up their control units widely in standard diagnostics, others very little. On many recent cars, a central gateway filters what goes through the port.
- The protocol: on a car that does not speak CAN (most models from before about 2008), the scan of the other control units does not apply. The app says so. See Is my car OBD2?
- The adapter: to reach a specific control unit, the adapter must accept address settings that many cheap clones ignore while still answering “OK”. On the same BMW, a generic Wi‑Fi ELM327 found 2 control units, a Vgate vLinker 18. Our Which OBD2 adapter to choose page details these tests.
Module by module: what you can expect
| Control unit | Standard OBD2 reading | “Control units” scan | What remains reserved for the manufacturer tool |
|---|---|---|---|
| Engine | Yes: codes, sensors, emissions checks | Yes | Advanced manufacturer values, adaptations, coding |
| Automatic gearbox | Often (if emissions-related) | Often | Learning procedures, resets |
| ABS / ESP | No | Depending on the make and the adapter | Bleeding the unit, sensor calibration |
| Airbag | No | Depending on the make and the adapter | Clearing crash data, module replacement |
| Air conditioning | No | Depending on the make | Actuator tests, flap learning |
| Instrument cluster, body | No | Depending on the make | Option coding, keys |
“Depending on the make” means exactly that: the only way to know on your car is to try.
What the app's “Control units” scan does
In DiagAuto AI, “Scan the control units” queries the possible addresses on the network one by one, reads the identity of each control unit that answers (part number, software version when it gives it) then its fault codes. Allow 1 to 4 minutes depending on the car. Nothing is changed in the vehicle.
At the end:
- you see the list of control units found, and which ones have faults and how many;
- you can read the details of their codes on compatible control units, without purchase;
- if no additional control unit could be read, the app says so clearly.
Some control units return codes in manufacturer format, which do not follow the standard P, C, B or U format: the app displays them raw and says so, without inventing a translation. When a standard code is found, “Explain with AI” gives the same explanation as for an engine code.
What an OBD2 adapter will not do
Even when a control unit answers, the app reads its information and its codes, nothing more: no reprogramming, no coding, no actuator tests, no ABS bleeding. These operations affect safety and require a manufacturer tool in the hands of a professional.
In practice
- Before buying a used car: the full scan gives an overview, useful on top of reading the engine. See Buying a used car: scan it before signing.
- ABS, ESP or airbag light on: standard reading will show nothing, which is normal; the control unit scan is the right door.
- Few control units found on a recent car: try a better-quality adapter before drawing conclusions. See Which OBD2 ELM327 adapter to buy and the list of features.
Frequently asked questions
Why does my adapter find 2 control units on my car and 18 on a friend's?
Three possible reasons: the car (some makes open up many control units at the port, others very few), the protocol (older cars without CAN do not lend themselves to it) and the adapter. On the same BMW, a Wi‑Fi clone found 2 control units, a Vgate vLinker 18.
Does reading ABS or airbag codes let me repair them?
No, reading a code tells you where to look. The repair (wheel sensor, connector, clock spring) is done on the car, and operations such as bleeding an ABS unit or replacing an airbag module require a manufacturer tool and a professional.