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Right diagnosis versus best guess and needless expense

By autotech-nath on April 11, 2022

In our ongoing series of case studies, courtesy of Darren Darling’s DPF Doctor Network, we hear from DPF Doctor member Keith Shanahan, of Shanahan Auto Services in Ballyedmond, Ireland. You won’t be surprised to hear that Keith is an advocate of getting the initial diagnosis right first time – on time, every time.

We had a couple of jobs in recently which once again demonstrated the importance of several factors that are sadly lacking in the many vehicles that come to us after they have been ‘seen’ by other garages. Correct diagnosis is dependent on a number of key areas including training, experience, correct tooling, technical information, product knowledge and, most importantly, the desire for a technician to seek an appropriate conclusion to a job – for themselves and for their customer.

Today’s case study involves an Opel (Vauxhall) Insignia. This vehicle had been to another garage. The owner was dismayed with the results after spending much time and money going back and forth to their workshop. He contacted me after reading about us on social media and locating us through the DPF Doctor Network website. Speaking to this gentleman, I gathered as much information as I could about the history of the vehicle, its issues, the work carried out to date including the extensive list of regenerations and the copious amounts of additives used.

As part of The DPF Doctor Network we go through a set of checks in an initial assessment. Part of this entails gathering key data, checking the physical condition of several components, checking for trouble codes and much more. Questioning the customer about their driving, fuelling, and servicing habits is also essential at this stage.

After completing the initial assessment, we had a lot of information to go back to our customer with and, to cut a long story short, we now had a good idea of the direction to go in regarding diagnosing the cause of our DPF problem.

We may well see one or indeed many faults initially but rather than fire in some additive or a fistful of parts we dig down to the root cause of the issue. Every time.

We may need some new parts, a repair of some sort, appropriate use of our go-to additives, JLM Lubricants, or even an “educational session” with the customer/driver. We never jump to conclusions.

In the case of the Insignia, this is what had happened elsewhere:
• It initially started off with the all too common “Fire in some

additives and drive the s**t out of it!” treatment.

• At no time was the customer asked about his driving style, distances regularly travelled, service history, fuelling habits, repair history etc. I knew all too well that the garage in question hadn’t done themselves or the customer any favours from the outset.

• Their initial course of action was unsuccessful. In fact, the car became more sluggish.

• The next step for this hapless garage? “Let’s put in some more additives and carry out a regen!” In fairness, this time around they did conduct a code read but disregarded everything that didn’t seem to be part of the actual DPF component itself.

• The visits to the garage became more frequent, the quantity of the additives used certainly outweighed the quality of the same and the regeneration of the DPF became a procedure that was undertaken multiple times during each visit to the garage.

Obviously, by this stage of the customer’s story, alarm bells were ringing. Later during the job, I discovered several issues that I knew would crop up after hearing all about it. Due to the excessive use of cheap additives, and the number of forced regenerations, the DPF and catalyst were thermally damaged. Worse than this was the fact that the initial cause of the problems was not the DPF itself, other much more affordable components had failed.

On this vehicle we discovered failed heater plugs and a differential pressure sensor fault caused by a wiring issue. Regardless of the number of times this DPF was cleaned, regenerated or soaked in additives, it would clog up time and time again because of these faults. Unfortunately, in this case it meant several bills for needless work and expense leading to the biggest expense of all for this customer – a new DPF.

The technician owes it to the customer to ensure they really are genuine DPF experts and skilled diagnostic technicians so they can perform a first-time fix and save their customer time and money.

For more information on how to diagnose and solve DPF related issues visit or



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Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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