Indeed, many main dealers do not retain good staff due to cost. They make the money on financing, above invoice & maintenance where the latter is a standard 100% markup on parts. A marque workshop manual which is basically a flowchart of "check continuity, check signal in range, fix wiring loom, replace part A, replace part B, replace ECU, replace car". The code & part may actually be secondary to the cause of the fault and it is there where experience counts to avoid a costly spiral of parts & labour
On fault codes, always force them to come back. If more than 1 fault code, well advised to ignore and force one/them to come back. Certain cars are plagued by problems "well known" to everyone but the dealers. Good groups exist, try
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or one specific to your marque
Certain cars are affected by very expensive problems, beware before you buy. Common rail diesel promise high mpg, performance, perceived reliability. Very often they have incredibly expensive problems, costly maintenance. Dealer network lacks the knowledge and fobs people off with costly fake diagnosis until someone uses a marque call-centre "technical support database" whose first entry is "can we get the fault diagnosis outside warranty?". I recall a Mobil oil engineer stating he was not sure how future diesel engines would cope, because the carbon loading of oil was now beyond what oil engineering could handle. The end result being coking, lubrication failures of turbos, bearings & eventually reduced longevity of engines. All it takes is a slight problem, and engine viability can be compromised. That was in
2000 and quite a few CR diesels are proving far less reliable than their previous incarnations. R&D may solve the issue, but it could be an expensive one for the buyer due to failure modes.
Many cars are increasingly trying to "proprietarise" their maintenance & repair. OBD-II readers are fine, often just different plug adapters required. However some ECU are conformance coated (rendering repair impossible) or even epoxy encapsulating, which I could hardly believe considering their thermal dissipation until I saw it myself. That rules out cheap repair solutions, and the ECUs no longer cost =A3350-650 but in some cases quite far into 4 figures.
Remember to counter any fob offs under warranty. Get any problem investigated & written down on invoice. If there is a major failure out of warranty, you can then claim back as the problem pre-existed under warranty. That whine could turn out to be a turbo failing, differential, gearbox and labour plus parts can be eye watering.
Cars are being treated as a "flash upgrade & fix appliance" with R&D continuing into ownership, and trying to proprietarise marque service networks. So beware the flashy car going cheap, sometimes the market prices them cheap beyond mere "not fashionable or VED".
DIY, absolutely.
The problem with the UK is high baseline cost of living which means the pros are seeing profits erode & cowboys whilst charging less are making so little they are not able to do a proper job. That is compounded by not so much the scarcity of skills, but the most common skill being incompetence. There are some very skilled tradesmen, but many who are not which merely devalue the work. Education is not helping, we seem to be migrating to standards that are lower and taking many more years to achieve - erosion on two fronts.
Still if you view the West as "run for what makes bankers richer", then you will soon realise that its population is expendable because it can be substituted by that elsewhere. So far that has been "making & service", the real hit will come when that population elsewhere becomes the "spender" where at present it is somewhat constrained (India & China). They the West will neither need people as skilled, nor as consumer, and that could see living standards peak for following generations (if it not already has).