Definitive Diagnosis

Vehicle diagnostic blog


In the the last few months I have found myself studying electronic circuitry to get to the bottom of some complex vehicle faults . Now a few years ago I would never imagined I would have an understanding of transistors,resistors,diodes,capacitors and how they contribute to a working circuit.

Now I am no expert on the subject and I have so much that I still would love to learn about , But taking my knowledge of electronics to the next level has been hugely beneficial to my understanding of vehicle diagnostics, the way I carry out tests with the oscilloscope and multi meter and also why components or circuits fail in the way they do.

I have written this month’s blog in the hope that someone who enjoys diagnostic work as much as I do might be inspired to further their own knowledge of vehicle electronics .

One of the circuits that inspired me to dig a bit deeper into this subject was explained to me by John Batten of AutoIQ in Northampton , and it is the H-Bridge circuit .

The H-Bridge circuit has so many applications in vehicle electronics , Before I had this explained to me I didn’t even realise how many components can be controlled by using this design. The throttle body motor is probably the most obvious use , but other applications can include EPB motors, Power windows, HVAC motors, Seat adjusters and many more. I am not going to go into too much detail as like I said I am no expert and there are many sites that will explain it in great detail far better than I ever could , But take a look at the circuit below and the waveform of a throttle body sweep from closed to wide open and back again. You can see how the path of current is controlled by the transistors switching and there are diodes ( or one way valves) to stop back emf from delivering any voltage created by the motor moving in reverse(generating voltage) back to the computer. Knowing how that motor is now controlled inside the ECU helps understand the waveform and makes a definitive diagnosis of this component much easier.


Another circuit design I have been studying is the Wheatstone bridge (or Null comparator) , this is used in Thermistor and Transducer circuits . It uses Ohms law to measure resistance very accurately which gives more precise feedback to the processor of temperature or pressure.


I have also spent a lot of time looking at Pull up and Pull down circuits which is knowledge I now use on a daily basis when doing circuit tests . Again if anyone reading this does not know what they are or how to determine which is which please take some time to find out , I promise you the next time you are diagnosing a possible sensor issue on the vehicle and you understand how these circuits work it will make it much easier to set up simple tests and manipulate the circuit to confirm a 100% accurate diagnosis.

ECM Pull down circuit

If you know all about these circuits and understand them fully then please share that knowledge with those who might benefit, After I was inspired to dig a bit deeper my knowledge has expanded greatly and I really hope my blog inspires even just one other person to become even more passionate about the subject and use it as a stepping stone to the next level.

Glenn Norris CAE AMIMI


No fault codes , Now what ?

The vehicle in this article is a Citroen DS3 1.6 Diesel and after a brief chat with the customer regarding driving style,service history and journey types we then discussed the symptoms of excessive black smoke when the revs were increased .

Driving the vehicle into the workshop confirmed the symptoms the customer described , thick black clouds of smoke were clearly visible.

I carried out some visual checks in the engine bay , oil level was perfect and also coolant level was as expected. Next step was to interrogate the ECU for fault codes , there were none .

If you have read my previous posts on this blog you will know I am a great believer in process , which often involves stepping away from the vehicle for 10 minutes and building a test plan so I am not aimlessly testing components in the hope of stumbling across the faulty part/s . So this is what I did next , writing down the possibilities of what could be causing the smoke and what tests/data I need to evaluate in order to either prove what is working or what is not working as it should.

On a vehicle which has only done 30,000 miles and had dealer servicing and mixed driving conditions , I was going to be looking at Fueling , Air intake and EGR as all other possibilities as the other options I would not expect to see on a car in this condition.

First thing to check was fuel rail actual and nominal values in the data from the scan tool against engine speed , cranking and idle they were identical with good cranking speed and under load also as expected. after raising the rpm and left to idle I switched the engine of to observe the speed at which the rail pressure dropped as this is a great indicator of injector leakage, the pressure slowly reduced over about 8 – 10 seconds which i was happy with . Also a quick check of the injector quantity comparisons confirmed that I was not looking at an injector fault at the moment.

Next up was to check the intake for leaks with a smoke test and this checked out absolutely fine so I now know we have no false ambient air .

Another parameter I checked was Cam Crank synchronisation which could point to a timing/pump problem , this also checked out ok.


So far I was happy that I had ruled out a few of my initial possibilities quite quickly which left EGR and mass air flow data. MAF was my next test and this was the first piece of data that indicated an clear issue . I monitored the actual value against the nominal value and there was a clear deviation between the two (seen below)


This proved that the volumetric efficiency of the engine was being comprimised , so that led me into my next test , if it was not false air(which I had already proved) then it had to be EGR ! I took at look at the data (seen below) which proved further investigation was needed for the EGR system as the actual value for EGR position was 255% and at idle should be 0-2%  !!!


After a chat with the customer explaining my findings the EGR was removed ,inspected and renewed and the vehicle was now back to its former glory with no smoke .

I hope this shows that without any fault codes all that is needed is a logical thought process which usually involves proving was is working to find what is not working whilst gathering evidence to warrant removal of a component.

Glenn Norris CAE AMIMI

Definitive Diagnosis

Pico Scope Circuit test using Ohms and Watts Law

Now I know this test will not be for everyone as we all have different processes and tooling to carry out the same tests but I hope this can show good use of Ohms/Watts law using the Pico scope to test a complete circuit using just Battery voltage on Channel 1 and Amps on Channel 2 and takes less than a Minute to set up.

Once you have the 2 channels hooked up for volts and amps you need to create 2 maths channels to calculate Ohms Law and Watts law

Click the tools tab(pictured) then select ‘maths channels’

Screenshot (298)

A window will open which has some built in maths channels to select from , we are going to use A*B and A/B

This will give us 2 extra channels – one will use Ohms law to graph channel A(volts) divided by Channel B (current) equalling Resistance

The other will show Watts law using volts times current equalling power(in watts)

Screenshot (301)

Now when you click start you should have 4 channels instead of 2 and it will now do all the maths for you !!!!

The Next picture shows a lighting circuit being switched on and then off again so we are testing a loaded circuit and can now see Volts ,Amps ,Resistance and Watts

Screenshot (295)d

In this we can see only 11 watts which is very low for a headlamp circuit and also 13 Ohms of resistance which is high for this circuit(enough to stop the lamp illuminating) , also a very low amp draw.

That’s 3 valuable measurements for any circuit and now gives good reason to look for a resistance in the circuit.

The last picture shows the result after removal of an 11 0hm resistance , Resistance is down to 2 ohms , power is fully restored(watts) and an expected current is flowing again.

Screenshot (297)s

Imagine you find a 5 ohm resistance in one wire – You know there is still another resistance in the circuit which can save some head scratching later on!!

Hopefully you can find a use for this and it helps along the way , It can be used in so many applications (actuator circuits , lighting , etc )

Glenn Norris CAE AMIMI

Definitive Diagnosis

Back to Basics


It can be very easy to get caught up in technology and over think your routine/process in vehicle diagnostic’s , when sometimes all is needed is to take a step back and concentrate on the fundamentals . No matter how simple or complex the system , a solid understanding of how something works , Why it has failed and what effect that could have on other parts of the system is an absolute necessity , and in this case study I found myself going ‘Back to Basics’ to get answers .

The vehicle in question was a 2000 Jaguar XJ8 Four Litre V8 and I was asked to investigate symptoms of rough running and inability to rev over 3500rpm and the idle was very erratic.

Having road tested the vehicle to confirm the symptoms I proceeded to connect the scan tool to the vehicle to interrogate for trouble codes. I have various scan tools available but only had the basic OBD II available for this vehicle across all platforms. This did not deter me from following through with a diagnosis.

I was presented with two trouble codes

P0302 ­- Misfire cylinder 2

P1646 – O2 sensor bank 1 heater circuit

From here I quickly confirmed a failure of the ignition coil on cylinder 2 and carried out the necessary repair, then continued to test and replace the O2 sensor (which was on the same bank as the failed coil) cleared the trouble codes from the Control unit and started the vehicle

The V8 fired into life and settled to a very steady and smooth idle, Job done I thought…

I proceeded to rev the car but it was still not happy, it coughed and spluttered its way up to 3500rpm on wide open throttle yet was so smooth on idle. I rescanned the for trouble codes and there were none , so I had rectified two faults (idle issue and O2 code/eml) but obviously still had more to deal with.

With no trouble codes and very limited data values to monitor I decided I needed to step away from the vehicle and plan my next test as it was getting late in the day.

The next morning I started the vehicle from cold and much to my surprise it revved beautifully right through the rev range! After a brief moment of excitement it returned to its previous state, idling beautifully but spluttering on wide open throttle.

I let the vehicle cool again and monitored the limited data values available from the OBD II list.

I had spent time testing the O2 sensors previously so was happy that these were not the cause in a closed loop situation. The throttle position sensor was not showing a signal from cold start so I tested this next with the oscilloscope, it was working perfectly.

So what was I missing?

This is where I went back to basics …..

Knowing the engine internals had the capability to rev due to the cold start I was left with the absolute basics ……… Fuel and Air!

Looking at the fuel trims I soon found I had a lean condition and Rich command  , The OBD II data has these very powerful PIDs which are a great indicator of what is going on inside the engine and also how the ECU is reacting to its condition.

Fuel pressure checked out perfectly at 2.6bar and 3.0bar without vacuum so I went after air, a quick smoke test of the intake system proved there were no leaks so I tested the MAF sensor to confirm it was providing the ECU with the correct information (which it was).


So fuel pressure was good, Air was being monitored and flowing perfectly

What was left?

Coffee and some brainstorming with a colleague, Fuel flow or lack of it, was discussed, and also the question….

How can that be proved?

That was the key, the vehicle was revved with an accelerant introduced during WOT …… The result was a clean sweep through the rev range. So although the Fuel rail pressure was within manufacturer’s specification the flow through the injectors was not enough for perfect combustion to take place.

With a new Filter fitted the vehicle was much improved proving the pressure was good but flow was indeed the issue , from here the customer took the option of a fuel additive program , due to lack of funds to restore the rest of the system to its former glory .

Now to some reading this , you might think , there are tools that test for that(injector test benches , flow testers) , but I have written this to show that with a logical thought process and using limited data/tooling , difficult problems can still be overcome by using what you have to its full potential and spending time understanding the fundamentals .

If it was easy, it would be no fun

Glenn Norris CAE AMIMI

When a misfire is not JUST a misfire

I am a great believer in questioning every step of my own diagnostic process from start to finish to be sure that next time I am presented with a similar fault I will be more direct with my process and achieve that all important first time fix with minimal effort and minimal expense for the customer.

During this particular diagnosis of an Audi A4 3.2 V6 that presented with an EML on and hesitation on acceleration , It could have been very easy to accept that testing the ignition coils (of which 3 were faulty)and then replacing a new set along with new spark plugs then road testing to monitor misfire counts would be enough?

This is the point I like to ask the question “Is there any possibility this vehicle will return for a related fault in the future?”

One of the most overlooked parts of a scan tool is the OBD readiness tests,combine this with a full drive cycle and you get an answer to that question.

After carrying out the drive cycle I was presented with a new fault code that I had not seen before 

17549 Load calculation cross check – implausible value.

With no dealer test plan available it comes down to understanding the description of a code and using logic to define why it has been set. In this case a load calculation taken as a measurement of the volumetric efficiency of the engine,so Manifold pressure cross checked against Oxygen content in the exhaust.

But why was I getting an implausible value after changing the coils and plugs?

I went over the possibilities in my head,had I nicked an air pipe? or was the ECU not able to set this code with a misfire and there was another fault present at the same time.

I set about finding out by carrying out a smoke test on the intake system to rule out leaks of which there were none. Then it was a choice between checking the Map sensor (no maf fitted on this model) or checking the O2 sensors.

Now using the initial fault as part of the diagnostic process for this new fault,the possibility of an O2 sensor being damaged by prolonged combustion failure guided my diagnosis process towards the O2 sensor on the bank that the vehicle had most misfire counts. 

I ran an ageing test on the O2 sensor and there it was. A quick ageing test would give me a reason to pull out the scope and confirm 100% that this was the cause of the new fault code.

With a Genuine O2 sensor fitted,another drive cycle completed and all readiness tests completed by the ECU the vehicle could now be returned to the customer knowing that it was a definitive diagnosis.

Glenn Norris CAE AMIMI

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