Diagnostic plug in thingys

[POMMReg]

Anyone have any idea how faults are detected using these checks?

Should the "fault" be picked out ON the VDU,or on the print out?

Any help greatly appreciated.

[The vast minority]

Search eBay for OBD or OBD2 depending on requirement. Google is your friend here. Faults are detected by sensors measuring presence of things like magnets and oxygen and stored in the cars computer. Other sensors measure vibration and throttle position and all add insane levels of complexity to what is essentially a very simple thing. Indication of a fault is provided by means of a light or a flash code depending on decade or century. If an engine light is fitted you are looking at OBD potocols and readers are available for a few quid at a car spares store or online.
my minor isn't so equipped as OBD was experimental only in 1964 and would have required a reel to reel computer on the back seat just to tell if the lights were on or off.
However, systems did exist to assist in fault finding even then. For example, 2 sensors located on each side of my 1970 head would detect if the crank had snapped as it would get very noisy. Additionally, there are 2 optical sensors fitted to the front of the head to warn of overheating. Often this manifested itself by the egress of steam and was viewable from the driver seat but could be further endorsed by the system upon undertaking a quick walk to the front of the car to check for bent grille and dead pigeon wedged in the radiator. Rudimentary systems also exist on my car (all factory fitted) to detect low fuel, failed ignition and even a dead battery. This is all categorised i under the CWG protocol (car won't go) I believe my modern cars are equipped with a yellow engine light that warns the operator of issues like these with the added bonus that you can plug an OBD2 reader in a convienient port under the dashboard to confirm a range of faults without use of a VDU.

[tysonn]

Very often responsible for erroneous and expensive error readouts.My CRV with LPG conversion recently showed an engine management problem warning light.Took it to my mates garage and the code reader said it was the knock sensor.He cleared the warning light and after a bit of revving etc when he thought it sounded ok we decided to leave it mainly because I'm tight.Two days later light was back on.I had a google around and several people said unscrewing the petrol cap had cured the fault.My LPG system starts the car on petrol and auto switches to gas within a couple of minutes so I tend to just keep the petrol tank half full,the last time I put petrol in was maybe two months ago.I switched off,unscrewed the cap and there was a loud inrush of air and the light went out!!!!Hasn't come back on since.
Mick

[warweezil]

The Megane 2 owned by my sister had an ABS fault indicated by the warning light, it took 3 different diagnostic units to arrive at the conclusion that the O/S/F sensor was the root of the problem, so not all this kit is equal. I have a small palm sized unit that does basic code reading but it took a big (and rather expensive) Snap On unit to get to the heart of the issue.

When I was at a VW dealership in my teens (Job creation thing) there was a diagnostic unit specifically for VW, that took up the entire length of the wall beside one of the lifts. It had quite limited functions and was hated by most of the mechanics there.

I guess in years to come the current system of pug ins will be as obsolete as the old Crypton/Sun Diagnostic units, cars will probably communicate with a central server and be booked for repair automatically as by then it will be almost impossible to do any work on them without a huge array of manufacturers kit. I hear there is now an Audi that requires "electronic intervention" even to just change the brake pads - crazy!

[tysonn]

Thank God I'll be long gone by then!

[POMMReg]

The full story is my Aunt's Peugeot 406 started to "leapfrog" - suggesting to me
a fuel problem - it was given a diognostic check and £215 later the problem was
still present. It was put through another check and the TRUE problem - some
part linked to the ABS costing £27.
So, as the initial work was unnessessary, can the cost be recouped?

Thanks

[TFM150K]

I would suggest that the answer to your question has nothing to do with Diagnostic plug in thingys and more to do with the determination of the Aunt to get her money back.
Good luck to her!

[panky]

Was it the diagnostic check that cost £215 or the original repair they did to 'fix' it.

[POMMReg]

Think the test was £70, the "remedy" faux pas, parts,labour and
Custard creams came to over £400!

I'm all for going back for a refund, but where does she stand
legally or otherwise - surely she should be entitled to the dosh
paid for parts/labour of the initial bill?!

Thanks

[chesney]

It's a slippery slope.
A friend of mine has a 54 plate Mondeo. About 6 months ago it cut out, on the way to York, and promptly wouldn't start again. Green Flag suggested it was the fuel pump. Took the tank out, replaced the pump, no better. Bled injector rail, no better. Had the injectors replaced, no better. Took it to a diagnostics place, who couldn't give him any answers. By this point it would run on the 20th or so try but cut out after 5-10 mins. Took it to a more renowned diagnostic place, who could only tell him that they thought the replacement injectors were too long and had pierced the pistons, which is tosh as it ran and was the same regardless of injector change. Changed fuel rail sensor, and finally the clue was the crank position sensor. When we took it out it was covered in 'iron filings'. Replaced that, and took it to a garage who told him (with an endoscopic camera) there was a tooth missing from the flywheel, which was 'confusing' the crank position sensor. He'd had the clutch and flywheel (think it was a dual mass one too) changed 3000 mile/4 months before. So, complained to the garage who fitted it, they blamed the supplier, then the supplier blamed the fitter back, in fact I'm not sure it's settled yet and he's lost the thick end of £1000 in parts and diagnostics. May not relate to your case, bit it seems to me it's difficult to get money from garages at all, so good luck!

[MarkyB]

If you don't fancy the expense of taking the car to a main dealer the best approach is to use a cheap reader to find out the code that's being put up, then look at the forums for the symptoms the car has.
Don't forget to check out related vehicles that are the same car with a different body/makers badge.

These are the sort of codes that you get;
P0070 Ambient Air Temperature Sensor Circuit
P0071 Ambient Air Temperature Sensor Range/Performance
P0072 Ambient Air Temperature Sensor Circuit Low Input
P0073 Ambient Air Temperature Sensor Circuit High Input
P0074 Ambient Air Temperature Sensor Circuit Intermittent

Read more at: http://www.obd-codes.com/trouble_codes/
Copyright © OBD-Codes.com

Interpreting them can vary between different makes I think.

[kennatt]

indeed as above,if you go on line and to the various forums,there is invariably a guide to how to trigger the system to give the codes on the flashing warning light,I have a recurring management light fault on my wife's nissan. A bit of welding wire across the terminals on the diagnostic plug,flashes the light and the code can be read via a chart on the forum. Then the light can be cancelled the same way.
As far as electronic intervention, as mentioned above,many moderns now require the ability to read the management system and reset it before and after any work.My brother has had to buy equipment ,for his workshop,at stupid cost because new cars require the system to wind back the brake pistons to change the pads,there is no other way of pushing them back into the caliper.Then the light comes on if you don't tell the system to accept the new pads. Absolute madness.So DIY forget about it.