Your pass to diagnostic success

By John Batten |

Published:  22 January, 2020

Would you like to get rid of that sinking feeling? The one where you fit a part and the problem still exists? If so then just keep reading

Hands up; Who wants to reduce levels of diagnostic stress in their garage? If that sounds like you then this article will be right up your street.
The pressure that a technician or garage owner experiences when a job doesn't quite go to plan can upset the apple cart and ruin your day. This month, we take a look at what can be done to not only reduce diagnostic stress, but dramatically improve your chances of a first-time fix, while driving down diagnostic time. What’s not to like about that? Nothing, so let’s get started.
This month’s narrative like all good diagnostic adventures is filled with twists and turns. You’ll no doubt nod along having trodden a similar path at some point in your garage. The good news though is that while there may have been angst along the way, this tale has a happy ending. All being well, you’ll be able to implement change so that you, your technicians, and you garage feel less stress.

Once upon a time...
Steve (name changed to protect the innocent) owns a great garage and like many of our clients cares very much about his customers. His objectives are straightforward, Steve wants to 1) have happy customers, 2) have a profitable garage, and 3) have a happy team. Sometimes though trying to keep a customer happy can get in the way of points 2 and 3 as you’ll see on this occasion.
Steve was presented with a petrol Vauxhall Zafira. The mission was to be diagnostic battle. The customer complained of a lack of power and the MIL light being on. Steve being an avid fan of our 15-step diagnostic process rolled up his sleeves and went to work. Here are the highlights of what he found:

  •  The vehicle on road test did indeed lack power
  •   Fault codes were present for lean lambda readings
  •  Serial data confirmed lean lambda codes
  •  High positive fuel trims attempting to correct lambda data

With the data clearly pointing towards a fueling issue Steve carried out a bunch of tests and discovered:

  •  Gas analyser values showed a very lean mixture
  •  Fuel pressure and flow were good
  •  The vehicle had no air leaks
  •  The vehicle had no breathing problems
  •  The MAF sensor was reading low and faulty.

So far so good
Steve at this point was feeling comfortable with the way his diagnosis is going, who wouldn’t be? This vehicle just needs a new MAF sensor, right? Well, Steve thought the same. Now, this is where it all gets a bit sticky.
Steve ordered a new MAF sensor, fitted it to the vehicle, and re-inspected his data values. The emotions that Steve felt were not ones of joy but anxiousness and frustration. The data was still wrong. Not only was the data wrong but the lambda sensor was now stuck rich and the tailpipe emissions were a particularly high eye watering 9% CO. Steve’s day just took a turn for the worse.

It’s good to talk
At this point Steve picked up the phone and we had a chat. We diligently went over Steve’s diagnostic process which was good, it had to be the MAF sensor!
“Just one thing” I asked; “Is it a genuine sensor?”    
“No” Steve replied. “It’s an old car, I’m trying to keep the cost down for the customer, an OE sensor is almost £250! I’ve fitted and aftermarket sensor.”
I completely get where Steve was coming from. Steve’s client was a long-standing customer with limited funds and a vehicle that was barely worth repairing. Steve was being a ‘nice bloke’ and attempting to do right by his customer and keep the cost down. We’ve all been there and no doubt been bitten by the results at some point.
I outlined that we’d need to fit that OE sensor before we could move the diagnosis forward. Steve at this point was naturally hesitant, “How do we know it’ll fix it?” he asked. “Don’t worry” I said, “We’ll prove it before you order and fit it”.

Your pass to success
Like many things in life, where there’s success there’s process, and quite simply a crucial part of any diagnostic routine is confirming the fix before a part is fitted if possible. We call it a bypass test, it’s a thing of beauty and not only confirms your diagnosis, it immediately reduces your stress levels!
Here are some points to consider prior to carrying out a bypass test. First, a bypass test is implemented after relevant tests have been carried out and you’re sure it’s a component at fault.
You must also understand how the sensor and the ECU circuit operate. You then build a circuit and apply it to the vehicle to simulate the component.
Sensor simulators can often be used but not for all situations. Lastly, but most importantly, always remember that a lack of understanding often results in smoke

Let’s keep it simple
What’s required to simulate this particular MAF sensor? Nothing more than a varying analogue voltage on the signal wire between 0.6 volts and 4.5 volts dependent on the air mass drawn in. How is that created? Like this; see fig. 1.
A straightforward voltage dividing circuit can be created from a few pounds worth of electrical components, or a sensor simulator can be used. As time was of the essence Steve borrowed our sensor simulator. Most simulators are capable of creating a range of analogue and digital signals for the common vehicle sensors so it was just a case of Steve selecting the right option and dialing in the required voltage.
Steve dialed it in and was overjoyed to see the behaviour of the data. Lambda was now in the correct range, and emissions values were spot on. Armed with this information Steve had the confidence to fit an OE sensor and fix the vehicle.

From stress to success
What lessons can be gleaned for Steve’s pain? An OE sensor fixed his vehicle; Having a test plan leads to correct diagnosis; Bypass testing prior to component replacement confirms diagnosis; Bypass testing gives you confidence to buy expensive parts; Bypass testing reduces your stress.
There’s a lot to like about a bypass test. It’s worth bearing in mind though that a lack of knowledge here can result in blown components, fried ECUs and the production of smoke. It’s worth gaining the knowledge on when and how this can be used in your diagnostic process. Used correctly though it’ll drive down the time it takes to get to a first time fix and reduce your stress.

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