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Generic OBD Code Readers

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Cheap OBD Code Readers

It's a fairly common phrase to be heard said by a car owner with a problem.
"I'll buy a cheap code reader and then..."

Let me stop you there.

Something is only cheap if it works.

£10 might be cheap for a code reader but if all it is good for is taking up space in a drawer, it's not cheap.

It Depends On The Car

So far, the smart range has used 3 different protocols for the OBD.
The 450 and 452 Roadster used K-line.
The 451 and 454 Forfour used CAN.
The 453 fortwo and 453 forfour used UDS.

The problem isn't the different types of protocols as most generic code readers have different protocols built in.
The problem is how the K-line protocol was used by smart. They pretty much did their own thing.

UDS is attempting to become a set amalgamation of K-line and CAN. Only time will tell on that one.

450 Fortwo & 452 Roadster

K-line was the 1st widely used OBD protocols. For the cars at the time, it worked fine.
It was introduced when cars started using ECUs (engine control units) and the car manufacturers
wanted a way to be able to diagnose problems with the ECU and the engine using a computer.

The problem is that K-line was insufficient for what came next.

K-line is a single bi-directional wire carrying data from a single unit. However, as car manufacturers
progressed, they added more and more electrical units to control parts of the car. Some car manufacturers
even put 2 ECUs in to control an engine. To counter this problem, some cars had 2 K-lines on one OBD port.

As manufactures tried to work around the limitations of K-line, the standard got ignored and they
did their own thing. Smart was one of these companies but they took it to a whole new level.

This is what standard K-line should have been.


4 - Ground
7 - K-line ECU
16 - Permanent Live

However, smart entered the arena on day 1 using this layout.


1 - K-line SAM unit/Zee Unit
3 - ECU data
4 - Ground
5 - Ground
7 - K-line ECU
8 - Switched live 12v
9 - K-line ESP/ABS controller
11 - K-line PAS controller
12 - K-line Airbag controller
16 -  Permanent live 12v

So you can imagine that there aren't many code readers that have 5 K-lines built in just for smarts.

So, if you buy a cheap code reader and use it on a 450 fortwo or 452 Roadster,
you will only be able to read error codes in the ECU and no other control units.

You won't be able to read airbag error codes so you can't delete them so you can't turn off an airbag light.

451 Fortwo & 454 Forfour

Luckily, a new protocol was established before the release of the 451 and 454 forfour.
Car electronics had come a long way and manufacturers were desperate for an answer.

Bosch actually created the CAN bus in 1986 but it took a long time to be used due to the complexity and added cost.
Car manufacturers pushed their technicians to make the cheaper K-line system work for them for as long as they
could. Over time, the price came down and people learned how to use it correctly so manufacturers started to use it.

It took 22 years to make the CAN bus protocol mandatory on vehicles although
most manufacturers had been using it for many years before that.

The CAN bus isn't a single wire going to each controller individually.
It's a 2 wire network that joins every controller to every other controller.
This means that data can be passed on from all of the controllers to the OBD port.

What that means is, if you have a 451 fortwo or 454 forfour, cheap code readers should work.
However, the codes that it spits out still might not be of any help. More on that later.

453 Fortwo & 453 Forfour

In an attempt to merge and standardise the previous protocols, the Unified Diagnostic Service was created.
It is a 7 layer protocol based on an extended version of the K-line and CAN protocols.

It's all fairly new but although it is supposed to be joint CAN bus protocols, there is no evidence
to show that cheap CAN code readers will work, however, there is a chance that they will.

All we have to go on at the moment is that the old C3 and older version C4 diagnostic multiplexers
that smart/Mercedes use to communicate with the cars, don't work with the 453 range of cars.

An updated C4 multiplexer is required. This could be the protocol or it could just be the way that
the clone multiplexers have been made. Either way, it doesn't look promising for the moment.

Once UDS has been around for a while, cheap code readers will have to start using that protocol.

The Problem With The Codes

Ok, so you were lucky enough to be able to get some codes read from your car.
The problem now is that those numbers are not a set standard.

A standard was released but manufacturers were told that they could change what codes meant.
So, smart's diagnostics software might know that a code means one thing but if you look it up
on a list of standard codes, it might mean something totally different.

I've seen a smart complaining about an issue on cylinder 5. Talk about a phantom pain!

What Are The Options?

You can buy a code reader, hope it works, hope the code helps and fix it yourself.
Take your car to a smart dealer or smart specialist and have them use MB Star to get the real codes.
Buy your own MB Star and read the real codes yourself.

Are There Any Other Code Readers That Work?

Yes but they aren't cheap.
Foxwell, iCarsoft and Autel claim to make handheld readers that do specific things on the smart.
You'll have to read up on their specifics by doing a search online because their ranges change.


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