The OBDII Test – not the Messiah


OBDII Test – First tier diagnostics

Intriguing to read about all the hype regarding onoard diagnostic testers and wondered how it went this way. One thing is for sure, the OBDII tester in the hands of an incompetent mechanic can do more harm than good.  It cost me $4 000.00 after a nugget of ingenuity, the chairman of The Global Brain’s Trust misdiagnosed a stretched cam chain through some code or other  pulled out of a hat. That cost $1 200.00 – then came the 6 pints of oil used over 30 miles and a damaged catalytic converter. Engine rebuild, a further $2 800.00. What the hell happened up to this point I’ll never know but forgive I’m not – they were the official agents. So lesson learnt.

OBDII Test - the Auto Boss V.30
Auto Boss V.30 – wiki commons credit Florian Schäffer –

OBDII Test vs the Litmus Test

First of all the engine diagnostics tool better known as the OBDII scanner or CANOBDII, is not the only be all and end all of tools to have in your workshop. Sure, it makes life easier but it’s not the Messiah. Think of it as being a simple litmus test i.e. it can give you a general idea as to whether it’s OK. A better analogy would be the oscilloscope used in fault-finding or navigating by dead reckoning. If you don’t know what the wave shape is supposed to look like or your position is estimated you won’t know if it’s correct or not. Seasoned technicians usually have an idea as to what it should like, for instance the output of a Schmitt trigger, but if there is a glitch, is this normal or not? Yes, I have worked on a radar system which had a very strange looking pulse shape in the display circuit – without the manual I may thought it was abnormal. It wasn’t. (DR has been used for centuries, it doesn’t always have to be scientific though).



Diagnostics testers can have the user running around in circles. A student friend (accounting) purchased a $40.00 unit to diagnose a poorly running engine – after four hours he gave up. The workshop charged him $60.00 to rejoin a loose wire. This took them 30 minutes. Auto technicians usually have a good grasp of the data bus in a motor vehicle – they did after all study this. Most mechanics do not and to make matters worse, management let them loose with a scanner in your car. These testers are therefore only valuable to someone that knows what the OBD system is, what the codes mean and where the fault could exist, which still means a multitude of things.

OBDII Test – know your emissions.

The check engine lamp or malfunction indicator lamp is a bastion of the great USA’s auto manufacturing industry and the fight to lower emission. If the lamp is on or intermittently on it really tells you nothing except that there is a problem ‘somewhere’. This concept kicked in about three decades back and it still remains your number one friend, or foe. The OBD I and OBD II tests may show you that you have a misfire but not the component or loose wire causing the misfire. An interesting point here is that all cars have their quirks.  The old die-hards can trouble-shoot within minutes and remedy the situation. If you do not have the experience this is going to take many times longer. Google can be great for the lay-man but once it comes down to getting your hands dirty, this is another story entirely. On another note, many technically minded kids these days can set up and read diagnostic charts much faster than a 60 year old but the 60 year old will always outperform based on patience. We live in a ‘quick-fix’ world but for an internal combustion engine, there are usually no quick fixes. This means the diagnostics checker is only as good as it’s user. It may only cost 40 dollars, it may show you have a problem, highlighted by a trouble-code but what then?

Many years back a client popped into a motor mechanic friend’s home. He was driving his son’s car (normal aspiration- carb, no OBDII test needed) which was misfiring badly and could not idle.  The mechanic cross-wired the ignition leads – don’t try this at home. It cleared the carburetor of any gunk within a few seconds. Now this is something which most of us may not of thought of and it can be downright dangerous (think flame) but it worked. An old hand knowing his job.  No diagnostics tester will tell you what to do. And that’s the point.

The OBDII test is therefore not the Messiah, just a great and efficient way to put you on the right path. Like the analogy using the oscilloscope, it may have an output – what now?