Which adapter does my car need?

The most common question I get is not about fault codes at all. It is which piece of hardware to plug in. The answer depends on two things: what protocol your car speaks, and where its diagnostic socket is. A Bluetooth adapter is the right answer for most cars. A small number of older ones need a MasterLink and a specific lead, and a few cannot be read from a phone at all yet. This page goes through each case and matches what the car finder on my site tells you for your exact car.

Start with the year and the socket

OBD-II became mandatory on US passenger cars for the 1996 model year. It standardised the 16-pin socket and a small set of protocols for the emissions side of the car: ISO 9141-2 and KWP2000 on the K-line at pin 7, J1850 on pins 2 and 10, and later CAN on pins 6 and 14, which every US car has carried since 2008. If your car has that 16-pin socket and speaks one of those, a Bluetooth adapter reads its live data and engine fault codes.

Before OBD-II there was OBD-I, and it was not a standard in any useful sense: the connector, its location and the protocol all varied by maker. Many of those cars report faults by flashing a lamp. That is why the answer for an older car is often a different lead, or a code you read by hand.

Bluetooth adapters: Veepeak OBDCheck BLE+ and OBDLink MX+

The Veepeak OBDCheck BLE+ is the adapter I use on my own E39. It works with the iPhone app and with the Android web app, and it covers CAN and the K-line protocols. Check the label says BLE+: the app treats the plain OBDCheck BLE as a CAN-only adapter for newer cars. Let the app find it. Do not pair a Bluetooth LE adapter in your phone's own Bluetooth settings, because that alone can stop the app reaching it.

The OBDLink MX+ is the one I recommend for J1850 cars. J1850 came in two forms: PWM, the Ford standard, and VPW, the General Motors standard. OBDLink's own protocol table lists both for the MX+, and says its cheaper CX will not work on GM, Ford and some FCA cars built before model year 2008. Other Bluetooth adapters claim J1850 only in seller listings, and I have not run one on a J1850 car, so the car finder points J1850 owners to the MX+ alone.

Two things to know about the MX+. It uses Classic Bluetooth (OBDLink lists Bluetooth v3.0), not Bluetooth LE, so on iPhone it needs version 1.2.0 of The Analogue or later, and you pair it in iPhone Settings rather than inside the app. It does not work in the Android web app, which needs a Bluetooth LE adapter. That Classic Bluetooth path is new in the app, and I am still proving it on cars.

BMW K-line cars: the MasterLink and the K+DCAN cable

On the E34, E36, E38, E39, E46, E53, Z3 and Z4 E85, a Bluetooth adapter reads the engine's live data and OBD-II codes. The cluster, lights, immobiliser, climate and stability control speak BMW's own DS2 protocol on the K-line, and on cars like my E39 they sit on a different pin from the engine. A generic adapter never asks them. The MasterLink does, and that is the one MasterLink job proven on a real car: the body computers on my E39.

What a MasterLink owner needs beyond the unit:

• A K+DCAN cable. The BMW-style OBD cable the MasterLink plugs into, with the pin 7 and 8 switch. It is not included with the MasterLink.
• A BMW 20-pin to 16-pin adapter, on some cars. Every E34, E36, E38 and Z3, and E39, E46 and E53 cars built before late 2000, have a round 20-pin socket under the bonnet. Use a fully pinned adapter. Cheap partial ones leave out the diagnostic lines, and the car will not answer no matter what else is right.
• Power. On the common cars, the E36, E39 and E46 included, a good USB car charger in the 12 V socket inside the car does it. If yours has to live under the bonnet, my setup article covers that too.

The MasterLink is a K-line device. It has no CAN hardware, so on an E87, E9x, E60 or any F chassis, a Bluetooth adapter is what you need. Pre-1993 BMWs (the E30, E28, E24, E32 and early E34 and E36) use an older handshake that no Bluetooth adapter speaks. A MasterLink reader for those is in development, and until it works I will not tell you otherwise.

Pre-1996 VW and Audi, Porsche 19-pin, and cars you read by hand

VW 2x2. The Corrado and the 1993 to 1995 Golf and Jetta Mk3 have a two-plug diagnostic connector under the shift boot instead of the 16-pin socket. Their computers also speak KW1281, an older VW language no ELM327 adapter speaks. The MasterLink is built to read it through a passive 2x2 to 16-pin adapter, but I have not proven it on a car yet. Every year flashes codes too, which the app walks you through by hand.

Porsche 19-pin. From 1989 to 1995 many Porsches, including the 964, the 968 and the early 993, carry a round 19-pin socket. The engine, gearbox, ABS, airbag and alarm share one bus behind it, which no Bluetooth adapter reaches. The MasterLink is built to read it with a 19-pin to 16-pin adapter, and I am proving that lead on a car before I stock it.

Read by hand. Some cars only flash their codes: the Mercedes W124, W126, W201 and W140, the R129 to 1995, the Volvo 240, 740 and 940, the VW Golf and Jetta Mk2, and the 944 Turbo with the LED test socket. No adapter is needed. The app walks you through reading them, and the engineers explain each code.

ELM327 clones and other traps

Nearly every cheap Bluetooth adapter is an ELM327 clone. The genuine v1.4 firmware was sold without copy protection, so clones copy it, can report a version number that is wrong or never existed, and carry that firmware's known bugs. Many work. Some are marginal: one well-known adapter wedged on my E39 until I fixed my own app's K-line setup, and the same adapter worked in another app all along.

• On iPhone it must be Bluetooth LE. iOS does not let apps use ordinary Bluetooth serial adapters, so an adapter that only offers that kind of Bluetooth will not work unless the app supports it by name, as with the MX+.
• INPA, Tool32 and Deep OBD Bluetooth adapters will not work. Their firmware replaces the ELM327 command set, and they use Bluetooth serial, which iOS blocks.
• The chip matters more than the brand on CAN BMWs. Clones usually reach the extra modules but can drop some.

If yours connects and then answers nothing, open the adapter picker and tap My adapter is not working. It records what the adapter says about itself so I can see what you have.