Guide
How to price a diagnostic fee
Diagnosis is the most skilled part of most jobs and the part most often given away. Pricing it starts with treating it as work.
The short answer
Price it as a block of time at your own labour rate, plus the travel a mobile visit costs you. Decide how long a first pass diagnosis usually takes on the kind of work you do, multiply by the rate you calculated from your own costs, add the drive there and back, and that is the fee. It is arithmetic on your own numbers rather than a figure copied from anybody.
Charge it as a fixed amount for a defined scope rather than by the hour. Customers cannot judge whether four hours of diagnosis was reasonable and will assume it was not. A fixed fee for a defined first pass, with a clear statement of what happens if the fault is not found in that time, is easier to sell and easier to defend.
Then decide once whether the fee is credited against the repair if the customer goes ahead, and say which it is up front. Both answers are common and defensible. What causes arguments is deciding it at the end.
Why diagnosis gets given away
Two reasons, and they reinforce each other. The first is that the customer cannot see it. A replaced component is visible and obviously took parts and effort. Two hours of tracing an intermittent electrical fault produces nothing to look at, and the customer experiences it as somebody standing near their car.
The second is that mechanics frequently do not price their own expertise, because the thing they are good at feels ordinary to them. Finding the fault quickly is the accumulated result of years of work and a lot of equipment, and it presents as a few minutes with a scan tool.
The result is that the most skilled hour of the day is the one most likely to be unbilled, and that free diagnosis attracts exactly the customer you least want: the one taking your answer to whoever will do the repair cheapest.
Working out the number from your own inputs
Start with your labour rate. If you have not calculated it, the guide in this cluster on what to charge as a mobile mechanic has the arithmetic, and there is a labour rate calculator in the tools section of this site that does the same thing with four inputs. Do not skip this step and pick a diagnostic fee out of the air, because the fee is derived from the rate rather than being a separate opinion.
Then decide how long a first pass actually takes. Look at recent jobs rather than at the best case. For most people it is longer than the number they would have guessed, because it includes the road test, the reading of the fault codes and their history, and the time spent proving that the obvious answer is not the answer.
Multiply, then add the travel. A mobile diagnosis costs you the round trip whether or not the job turns into a repair, and that is the part shop pricing does not contain. If you already have a callout charge, this is where it applies.
The last input is your equipment. Anybody can read a fault code with an inexpensive reader. If your diagnosis depends on tools most people do not have, or on a subscription to manufacturer data, the fee is covering that as well and it is reasonable for it to be higher than one that does not.
Explaining it without arguing about it
The objection is almost always the same: they only want you to look at it, and looking should be free or cheap. The reply that works is not a defence of the fee, it is a description of what they are buying.
What they are buying is an answer. Which component, why it failed, what it will cost to fix, and what happens if it is left. That answer has value to them whoever does the repair afterwards, which is precisely why it is worth money.
It also helps to say what the fee is not. It is not a deposit against a repair they have not agreed to, it is not open ended, and it is not a way of getting them committed. Saying the scope and the limit out loud removes most of the suspicion the question carries.
Say the figure in writing before the visit. A diagnostic fee quoted verbally on the phone and then invoiced is the version that gets disputed.
Diagnosis is a product, and it has a deliverable
The single biggest change you can make to how a diagnostic fee is received is to give the customer something at the end of it. Not a verdict delivered while wiping your hands, but a short written result they keep.
It does not have to be elaborate. What the complaint was. What you checked and what the readings were. What you concluded and how confident you are. What the repair involves and what it costs. What happens if it is not done, and whether the vehicle is safe to drive meanwhile.
This changes three things at once. The customer now has evidence that work occurred, which is the missing ingredient in every argument about diagnostic charges. The repair quote arrives attached to a reason, which makes it far more likely to be approved. And you have a record of what you found, which matters if the same vehicle comes back with a related fault later.
Photographs earn their place here more than anywhere else in the job. A picture of the corroded connector or the reading on the meter does more to justify a fee than any amount of explanation, because the customer can see the thing they were told about.
There is a commercial side effect worth being honest about. A written diagnosis is also portable, and a customer can take it elsewhere. Some mechanics use that as an argument for keeping the findings vague. It is the wrong conclusion: a vague result feels like being kept in the dark, and the customer who was going to shop around was going to do it anyway. What keeps the repair with you is being the person who explained it clearly and can start tomorrow.
Whether to credit the fee against the repair
Crediting means that if the customer goes ahead with the repair, the diagnostic charge comes off the final bill. It is common, it is popular with customers, and it is worth thinking about properly rather than adopting by default.
The argument for it is that it removes the objection at the moment of booking. The customer is not paying twice, and the fee stops feeling like a toll on the way to a quote. It also biases them towards having you do the work, which is worth something.
The argument against is that it makes the diagnosis conditional. If it is credited, you are paid for your most skilled hour only when the customer also buys the repair, which means a correct diagnosis of something you cannot fix, or of something that is not worth fixing, pays you nothing. It also quietly rewards the customer who agrees to a repair they were always going to have done, and penalises the one who takes your honest advice not to bother.
A middle position some people use is to credit part of it, or to credit it only when the repair happens at the same visit. That keeps the incentive to book while still paying for the diagnosis as a piece of work.
Whichever you choose, decide before the customer asks and say it in the same message as the fee. The version that goes wrong is the one where the question first arises while you are writing the final invoice, because then whichever answer you give looks like it was chosen to suit you.
Where the short answer stops being the answer
Every rule on this page has a situation it does not survive. Here are the ones worth knowing about before you meet them.
The fault does not appear while you are there
Intermittent faults are the reason a diagnostic fee needs a defined scope. Say in advance what the fee covers and what happens if the symptom does not present, because that outcome is common and it is not a failure.
The deliverable matters even more here. A written record of what you checked and eliminated has real value on the next attempt, and it is what makes a second visit an escalation rather than a repetition.
The customer only wants the fault codes read
This is a different and much smaller service, and it is reasonable to price it differently or to decline it. A code is not a diagnosis, and reading one out has a habit of turning into an argument later when the named component turns out not to be the fault.
If you do it, be explicit that a code points at a circuit rather than a broken part, and that acting on it without testing is how people replace expensive components twice.
The diagnosis takes far longer than the fixed fee covers
Stop at the point the fee covers, report what you have found and eliminated, and quote for continuing. Working on unpaid is a habit that turns a defined fee into an unlimited one.
Getting approval to continue mid job is the same conversation as any other additional work, and it belongs in writing for the same reasons.
A customer wants a free quote and considers diagnosis part of it
Separate the two clearly. Quoting a job you already understand is free. Working out what is wrong is not, and that boundary is easier to hold when it is stated on the booking call rather than at the vehicle.
Where the complaint is a symptom rather than a known repair, saying plainly that finding the cause is the job being paid for usually settles it in one sentence.
When it has already gone wrong
Most people find a page like this after the fact rather than before it. This part is for them.
The customer refuses to pay because you did not fix anything
Point at what was agreed and at the written result. This is exactly why a diagnostic fee needs a defined scope and a deliverable: without both, the argument is one word against another.
If nothing was agreed in advance, this one is difficult to win and probably not worth the fight. Take the lesson and put the fee and its scope in writing before every diagnostic visit from now on.
You diagnosed it wrongly and the repair did not fix the fault
Say so quickly and directly. A mechanic who returns and says the first conclusion was wrong keeps far more customers than one who defends a diagnosis that plainly did not work.
Then decide what you are absorbing before the conversation rather than during it. Being clear about which part you are standing behind is what turns a bad outcome into a customer who still trusts you.
You keep waiving the fee to win the job
This usually means the fee is being used as a negotiating chip rather than as a price, and the customers who ask hardest get the discount. If it is waived for most people, it is not a fee, and the honest options are to remove it and raise the labour rate, or to keep it and stop waiving it.
If the reason is that customers baulk at it on the phone, look at whether they are being told what they get for it. Most objections to a diagnostic fee are objections to an unexplained amount rather than to the idea.
Questions
How much should a mobile mechanic charge for diagnostics?
This page gives no figure, because there is no sourced survey behind it and a made up number would be wrong for most readers. Work it out from your own inputs instead: how long a first pass really takes, at the labour rate your own costs require, plus the travel a mobile visit adds.
Should a diagnostic fee be credited towards the repair?
Both practices are common. Crediting it removes the objection at booking and makes the diagnosis conditional on the customer buying the repair. Not crediting it pays you for the skilled work regardless of the outcome, including when the honest advice is not to repair the vehicle. Decide once, and say which it is before the visit rather than at the invoice.
Why do mechanics charge for diagnostics at all?
Because finding the fault is the work. It takes time, equipment and experience, it produces a valuable answer whoever does the repair, and it is the part of the job most likely to take a whole morning. A free diagnosis also attracts customers who intend to take the answer elsewhere.
Is reading fault codes the same as a diagnosis?
No. A code identifies a circuit or a condition the vehicle noticed, not a broken component. Acting on a code without testing is how the wrong part gets fitted, which is why a code read and a diagnosis are usually priced as different services.
What if I cannot find the fault?
Say in advance what the fee covers and what happens if the symptom does not present or the first pass runs out, so that outcome is a known possibility rather than a surprise. Providing a written record of what was checked and ruled out is what makes the visit worth what it cost, and it makes the next attempt shorter.