No codes, no clues?
Forever the codebreaker, 2019 Top Technician winner Neil Currie shows what to do when there are no codes at all
Published: 17 March, 2020
Have you ever had a car in with a running fault or an issue, and you plugged the diagnostic tool into the OBD socket then read for trouble codes, only to be met with the message ‘no faults stored’?
For many reasons, this confuses technicians and stops them being able to progress with the job. They have no clues or starting point to work from. However, many other tests can be done to find the root cause of the issue. I have worked with many a technician who has been lost after finding a ‘no fault found’ message. I recently had a job where I was able to demonstrate to my colleague how knowing some numbers and how systems work and interlink can help identify what is wrong.
Call-out
The vehicle in question was a 2012 Land Rover Discovery 4. As we specialise in LR we have built up a good reputation in the area for being able to fix them, having also invested in dealer tooling and information. The customer’s first contact with us was via telephone and he explained he had parked the vehicle up outside his house and then having come to it the next day it would not start. The engine would turn over but it would not fire into life. He informed us his local garage had come out for a look and had been unsuccessful in finding the cause and recommended getting the vehicle recovered to us. He asked our call-out charge and asked for us to come and take a look before he organised recovery. This is not my favourite type of job as with limited tooling there is only so much you can do but we agreed to go and take and look and see what we could find.
No fault codes stored
Along with my colleague Jamie we went to the customer’s house that afternoon, taking a scan tool and the tool kit in our work van. Once we arrived we spoke to the customer to gather some information about the problem. He told us no recent work had been carried out on the vehicle and the other garage had done some basic tests on the battery and fuel system where it sat but could not find an issue. I sat in the vehicle and cranked the vehicle to verify the complaint, doing this also allowed a few checks to be done by listening to the sound of the engine cranking. A trained ear can pick up a compression issue, whether it is spinning fast enough or anything mechanical which doesn’t sound correct.
On this vehicle though all sounded ok. I then let Jamie do some checks to see what he could find. As a younger technician he mainly does MOT and general service work, so it was a good opportunity to possible teach him something along the way without the distraction of a busy workshop. After some basic checks he decided to plug in the scan took and see if any fault codes were stored. Upon carrying out a fault code report he was met with the message ‘no fault codes stored’. I then asked him what his thoughts were and where we go next. His reply was “I don’t know?” I am sure this has happened to some of you reading this article, we have all been there.
Live data
I explained to him that live data was a key element here and we should use it to our advantage. We need to look for data relevant to the complaint to rule out what it can’t be, and knowing what the numbers mean will do this quickly. Unfortunately, this takes years of looking at good data, taking notes and memorising it. Luckily for him, I was able to assist. My first checks were to be engine RPM, fuel pressure, immobiliser status, cam/crank synchronisation and a plausibility check of all temperature and pressure sensors to make sure they were in spec. Working through them all with ignition on, then cranking everything looked good so the engine should start but why wouldn’t it? This is where it pays to step back for a moment and evaluate what you know already and what you should do next.
Smoke/air pressure
An engine in its simplest form is an air pump. We know it needs compression, fuel and air to run. With what seemed to be good compression, and from what I had heard, also good data from the scan tool, with limited resources, I decided the next test would be to see if any smoke was being emitted from the tail pipes. This would show if there was any sign of fuel delivery to the engine. With good RPM and fuel pressure, if the ECU is happy, it should be firing the injectors. There was no smoke, however when I felt the tail pipes there was no air pressure whatsoever from either tail pipe. Was this a clue to where the issue may lie?
My first thought was we have a restriction and the engine cannot breathe, so we are missing the air section of the triangle for the engine to run. I then had a good visual inspection of the engine. Knowing the design well, I decided to open the inlet up to atmosphere by removing the map sensor to see if there was any change. If there was a blockage, this test would prove it and allow the engine to run. In this vehicle, the engine is a V6, so it uses a conventional V configuration. To allow air to flow into both intakes of each bank there is what Land Rover call an intake throttle manifold which also houses the MAP sensor, the EGR inlet pipework and a throttle butterfly flap with a rubber hose to direct air from the intercooler into the manifold (fig1). Removing the MAP sensor would allow air to be released if there was an issue from either EGR valve or upstream from the intake i.e. throttle butterfly, failed turbo just to name a few. On removing the sensor and cranking the engine it now fired into life and idled fairly well, this confirmed we had a blockage somewhere manifold side starving the engine of air.
Throttle butterfly flap
Checking the clock, we still had some time left allotted for the call out. I decided as it was easy to remove the intake hose to the intake throttle manifold just to see as a quick test if the issue was before or after. Upon removing the pipework and refitting the map, the engine no would not start, again proving the issue was on the engine side of the pipework. Removing the air intake plenum to the throttle manifold then revealed the issue. The throttle butterfly flap used to strangle the engine of air on shutdown had jammed shut and never reopened as the housing was heavily covered in carbon. This butterfly, when working correctly, should spring back open ready for the next engine start. Questioning the customer and his driving style revealed he mostly done slow speed and town driving and used supermarket fuel, all of which were a contributing factor to the issue as the valve sits closely to the flow of EGR gas from both valves. Forcing the valve open and refitting the components allowed the vehicle to be driven back to the workshop for a repair to be carried out.
Upon the removal of the entire assembly (fig2), it was found the unit would be better to be replaced as cleaning would not remove all of the carbon deposits and could cause the issue to re-occur. The EGR pipework was also removed and cleaned as a preventive measure along with an oil and filter change and the vehicle was returned to the customer.
Further learning
Why were there no fault codes stored you ask? Well on this engine the position off the butterfly flap is monitored and it should have stored a stuck closed fault but this may not be part of the software’s strategy so I am unable to answer why. However, this article shows that if you have an issue and no faults are stored, there are tests you can do to find the issue. So next time you have a scan tool connected, grab for example 10 good live data PIDs and store them then learn them off by heart. Once you have mastered that section move onto some more and soon you will build up a good mental library of what good data should be, which helps massively to fix cars!
- Common people
How do you go about diagnosing a common fault that you have seen before and all the symptoms match? Do you go ahead and fit that new part with no testing? Do you go straight to where you think the issue will be or do you test to be sure regardless of the situation?
You may or may not recall several articles ago, in the May 2020 issue of Aftermarket, I had a Land Rover Discovery 3 which would not start after being jump-started incorrectly and was fixed by reflashing the engine control unit software. Well, strangely enough, I was recently presented with a Range Rover Sport with near enough the exact same initial symptoms and fault codes. I want to show how starting afresh and testing, instead of jumping to the same conclusion, prevented a misdiagnosis.
Customer complaint
The customer’s complaint was that the vehicle would crank over but would not start. They said previously that the vehicle had started showing an intermittent no-start condition after sitting for a short period of time, for example to go into a shop. Once they returned, the car would crank and not start. The customer had discovered though that if they then waited five minutes and tried again, the vehicle would then start and be okay for the rest of the day. However, by now the symptoms had slowly become worse and no amount of cranking would start
the vehicle.
As always in my diagnostic process, the first step is to confirm the customer complaint and look for any tell-tale clues along the way. Yes it seems silly on the face of it to crank the engine over knowing it will not start, but an experienced technician may pick up a clue which will give direction where to go next so it always pays to always confirm the complaint. On this occasion confirming the complaint revealed no clues so it was on to the next step and to check for fault codes and review some live data.
Multiple fault codes
As can be seen in Fig.1, we have multiple fault codes stored for all different circuits and systems on the vehicle so where do we start? As in previous articles I have written, I always like to split them up into a list and put the most likely causes at the top and start there. Looking at the list we have five fault codes and I felt three could cause the no-start.
There are a number of likely causes. It could be a lack of fuel pressure, as the fault code states it is too low. The DC/DC converter fault also is another clue, as this converts the 12V supply from the battery and boosts it up to 60/70v to open the fuel injectors. The fact that code is stored could be another reason the engine will not start and the system voltage low fault code as this could indicate the control unit isn’t receiving the voltage it should to operate correctly.
The other two fault codes I felt could be put to the bottom of my list. An EGR fault most likely would not cause a no-start issue on this particular engine and there are two fitted due to the the engine being a V configuration. Having plenty of experience with this engine, I have seen many stuck open and closed EGR valves not cause the customer’s complaint due to the pipework configuration so it could be ignored for now. Lastly, there is the control box fan fault. This is a small fan mounted next to the engine ECU to control its temperature and would also not cause a no-start complaint.
Live data
My next step was to consult live data and look at module voltages and fuel pressure as these were at the top of my list. Cranking the engine while monitoring rail pressure showed there was next-to-no fuel pressure being generated, so this is one of the reasons the engine will not start. In that case, why do we have a low system voltage code and a DC-DC converter fault logged? With reference to Fig.2, looking in the module voltage section in live data showed why we have 0v for battery voltage and 3v for the DC-DC converter. As I mentioned, this should be around 60-70v on this particular vehicle so this explained the reason for the other fault codes. I then decided to pull up a wiring diagram and look at how the engine ECU was supplied power to formulate a plan of attack for these faults.
- Reasoning and diagnostics Part II
We began this journey last issue, so to recap: We need solid reasoning skills to carry out effective diagnostics; persistently good decision making doesn't happen by chance. Possibly out of convenience these skills are often underestimated and undervalued by people, both in and out of the trade. We must raise awareness of the discipline and precision of thought necessary for logical and critical thinking: so we can be better rewarded for our efforts; and to make sure they are consistently and properly applied.
Reasoning, arguments and hypotheses
We covered some fundamentals in my last article: we explain our reasoning using arguments, which contain statements supporting a conclusion; one type of argument, a deductive argument, should guarantee the truth of its conclusion (if it is sound); however, we need to use critical-thinking to check this, by making sure i) there are no other possible conclusions (which makes it a valid argument) and ii) the supporting statements are true.
- Building the better garage
The most important first step begins with vehicle and owner triage. Listen carefully, ask searching and relevant questions regarding the complaint, do not accept anything until you have confirmed the condition, and never accept previous work or opinion as correct. The customer must accept this cost or walk away. The triage may include, visual inspection, road test, or a preliminary global vehicle scan, i.e., all systems. None of this is free. It is part of a progressive methodical process. Agree a separate contract for this allowing either party to walk away.
One especially crucial point to understand before you begin any repair or diagnostic investigation, you must fully understand how the system functions and the specific responsibility of each component in that system, how it operates and how to test it.
Check DTCs that are relevant to the symptoms, not forgetting pending and confirmed errors in EOBD. Also check for incomplete default flags. These cannot be cleared unless all flag parameters have been satisfied during drive cycles.
Next, you need to cross-reference specified, actual, and corrected data. A fault code will not register unless the component parameters have been exceeded, in some cases for a considerable time, so fast intermittent drive concerns may not be registered in the fault memory. Previous experience over the past 50 years has convinced me of the value in using gauges when confirming, fluid, pressure, and flow.
For example, when testing fuel pump performance, flow is just as important as pressure. Also check the pump current. It is linear with pressure, therefore faults may be predicted by checking current across the relay or fuse without accessing the pump or supply hoses.
Intermittent variable vane turbo faults are easily monitored with a gauge. We could not source suitable gauges, so I designed our own. In fact, many of our tools have been modified to suit challenging tests.
Data log selected serial data so that focused analysis can be carried out. The selected items will depend on the nature of the fault under investigation. This can then be downloaded into graphing software like datazap. If you are interested refer to my VW Amarok SCR repair article from the June issue.
There may be a technical bulletin or software update dealing with the complaint so access to the manufacturers repair information system is mandatory.
Component testing
It is at this point where component testing may commence, like each stage of an investigation there are rules that govern and guide your response. Before the output of a sensor is suspected, you must check the ground reference and power supply at the sensor. Output deviation can be caused by wiring errors, sensor error, or a genuine environment value error. It may be necessary to cross-reference the value by alternative means, where possible.
For example, with a cold vehicle, all temperature sensors will have a similar value, as will pressure sensors on a static engine. Exhaust gas temperature sensors will reduce by approximately 50°C as they pass further down the exhaust stream.
Sensors fall into set groups; Position, range or movement, temperature, pressure, angle etc.
They also fall into three output categories; Linear/analogue, digital, and sent. Because of the complexity in vehicle systems control, it is inevitable that an oscilloscope needs to be used to confirm correct functionality.
An oscilloscope, like all tools, fall into one of three groups; The good, the bad and the ugly. They demand two special skill sets; Set up and image interpretation. They provide a unique insight to mechanical and electronic functionality.
This brings me to current and ongoing problems: Accessibility, and the cost risk ratio in the diagnostic process. Many of the tools that can be used with a scope find their roots in other hi-tech industries.
Cylinder pressure analysis, WPS, is the best example. The use of an absolute pressure sensor directly in the cylinder reports real time pressure differential above and below atmosphere. With minimal component removal and the engine running, the precise valve open/close position can be established.
The catch here is fully understanding the image as correct. It may require confirmation from a good known vehicle. The other problem is variable valve lift and timing control. This will affect pressure readings and must be confirmed via serial data evaluation.
Complex
Vehicles manufactured today are a complex mixture of mechanical systems, all of which share one unique property; Mass, acceleration, and frequency. The latest technique in systems diagnosis is NVH, or vibration analysis. With the aid of a three-dimensional accelerometer and analytical software, each individual component can be identified by its frequency signature. Everything from a cylinder misfire to a defective bearing can be isolated.
Some of our more individual specialist tools include an injector test bench. This helps identify combustion imbalance from our vibration analysis. With the onset of direct drive turbo actuator control, we invested in an actuator drive simulator. Driving the wastegate through precise angles whilst monitoring the current draw confirms correct movement, range, and mechanical resistance.
It is occurred to me writing this two-part piece that several subjects identified would make good subjects for future articles, so watch this space. It has also reminded me of the remarkable skills that automotive technicians need to repair and service vehicles. Have pride in your achievements and don’t work cheap!
- Snap-on: Vehicle system scan reports
The complete vehicle system scan reports offered by car diagnostic tools from Snap-on help technicians to get into good practices and give their customers peace of mind. Running a pre-scan before any vehicle work is done means any hidden issues can be located and the customer can be made aware of them from the outset. The pre-scan code identification helps speed the diagnosis and repair, and when it is followed by a post-scan after work is finished, the report confirms that all concerns were dealt with properly. Snap-on offers pre- and post-scan reports for technicians who own Snap-on diagnostic scan tools, including ZEUS, VERUS, MODIS, SOLUS and ETHOS families.
diagnostics.snapon.co.uk/vsr
- Lemförder switchable engine mounts
Under its Lemförder brand, ZF Aftermarket has introduced a range of switchable engine mounts for a number of Audi and Mercedes Benz models. Using an integrated air spring this can actuate two different characteristic curves depending on vehicle and rotational speed. The mounts are currently available for Audi and Mercedes-Benz vehicle models and includes the Audi A4, A5, A6, Q5, Q7 and Mercedes C and E Class, GLK, GLC. ZF Aftermarket recommends that all engine mounts always be replaced during repairs, not just the defective part. Otherwise, there is a risk that the new component will have to absorb higher forces and vibrations because the remaining mounts are already heavily worn which may accelerate subsequent damage.
www.zf.com
