Guide
How to handle customer supplied parts
The part is cheaper for them and the risk moves to you. A policy written down in advance is what puts it back.
The short answer
Decide your position once, write it down in plain language, and put it in front of the customer before the work is booked rather than after the part has failed. The three positions people take are: we do not fit parts we did not supply, we fit them under different terms, or we fit them case by case for customers we know. All three are workable. What is not workable is having no position and improvising it in a driveway.
If you do fit supplied parts, the terms that usually appear in a written policy are that labour is charged whether or not the part turns out to be correct or working, that the part carries whatever warranty the customer own supplier gives it and none from you, that a second visit to fit a replacement is chargeable again, and that you may decline once you have seen the part.
Say it before the job. A policy explained afterwards reads as an excuse, and the whole point of writing it down is that the conversation happens while the customer still has a choice.
What actually changes when the customer brings the part
On the surface it is a smaller job at a lower total. Underneath, three things move.
The money moves. Parts margin is a real part of what a repair earns, and it pays for the collection trip, the cash tied up in stock and the risk of a component failing. Fitting a supplied part removes that income while leaving most of the work in place.
The risk moves. When you supply a part, a failure is between you and your supplier and you can usually get a replacement without argument. When the customer supplied it, a failure is between them and a seller you have no relationship with, and you are standing in the middle of it holding the labour bill.
The diagnosis moves too, and this is the one people underestimate. A customer who arrives with a part has already decided what is wrong with the car. Frequently they are right. When they are not, you are being asked to fit a part that will not fix the problem, and the conversation about that has to happen before the work rather than after the invoice.
Why "we do not fit supplied parts" is a legitimate answer
Plenty of shops and mobile mechanics simply decline, and it is worth understanding why rather than treating it as unhelpfulness. Declining removes the entire category of dispute where a part fails, the customer is out of pocket twice, and nobody can agree whose fault it was. It also protects the margin the business actually runs on.
The cost of declining is real work turned away, and some of that work belongs to good customers who found the part cheaply and are being reasonable about it. That is why the middle position exists.
Whichever you choose, the important thing is consistency. A policy applied to some customers and waived for others stops being a policy and becomes a negotiation, and it is the customers who argue hardest who end up with the best terms.
Having the conversation before the part arrives
The conversation is short and it works far better on the booking call than at the vehicle. Three things to establish: what the part is and where it came from, that labour is payable regardless of how the part performs, and what happens if the part is wrong or faulty when it comes out of the box.
Ask them to have the part with them and unopened before you arrive. A mobile visit that ends with the part still in transit is an entire trip you may not be able to charge for, and it is a common way for a supplied part job to lose money before it starts.
Get their agreement in writing, which for most mobile work means the estimate the customer approves says so. An approval that includes the terms is a much better record than a memory of a phone call, for both sides.
What a written policy on supplied parts usually covers
This is a description of what these documents tend to contain, not a template and not a statement of what any rule requires. What belongs in yours depends on where you work and what you are comfortable agreeing to, and it is worth having somebody qualified in your state look at the wording once.
Whether you accept supplied parts at all, and any category you never accept. Safety related components and anything where a failure could cause harm are the usual exclusions, and stating the category in advance avoids deciding it under pressure.
That labour is charged for the work performed regardless of whether the part is correct, complete or functional. This is the clause the whole policy exists for, and it is the one most likely to be misunderstood if it is not written plainly.
Who carries the warranty on the part. Where a part is supplied by the customer, policies usually record that any warranty is between the customer and their own seller, and that the fitter warrants the workmanship only. Saying which of the two you stand behind is clearer than saying nothing and hoping.
What happens on a repeat visit. If a supplied part fails and has to be replaced, the labour is being done a second time, and the policy usually says whether that is chargeable and at what rate.
The right to inspect and decline. Policies commonly reserve the ability to look at the part on arrival and decline to fit it, with the visit still chargeable, because a part that is visibly wrong, damaged or counterfeit is not something anybody should be talked into installing.
How disposal and cores are handled. Old units, cores that carry a deposit and any hazardous waste have to go somewhere, and who pays for that is worth a line rather than an assumption.
What the customer is agreeing to and where they agreed it. A policy that lives on a website nobody visited is weaker as a record than the same words on an estimate somebody approved, whatever it says.
The failure everybody has the argument about
The situation is always roughly the same. The part goes in, the fault is not fixed or the part fails soon after, and the customer takes the position that they have paid for nothing. The argument is not really about the part. It is about who was supposed to know it would not work.
Three things reduce the frequency of it and they are all done before the part is fitted. First, look at the part before you start and say what you see. A part that is obviously for a different variant, is not the right revision, or arrived in a bag with no packaging is worth raising then rather than after the covers are off. Second, be explicit that fitting the part is not a diagnosis, and if you have not diagnosed the fault yourself, say so in the estimate. Third, do not let a customer talk you past your own doubt. If you think the part will not solve it, saying so and having that recorded is the single most useful sentence in the whole job.
When it happens anyway, the position that holds up best is the one you can point at in writing and have applied consistently. Take the part out or leave it in as you agreed. Charge the labour you said you would charge. Offer the practical help you can genuinely give, which is usually telling them exactly what the correct component is, so the second attempt works.
What makes it worse is arguing about whose fault it is. The customer chose the part and you chose to fit it, so the honest answer is usually that it was a shared decision made with incomplete information, and the paperwork is what stops that being a matter of opinion.
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 part is a used or salvage unit
Used parts carry a much higher chance of a second visit, and both sides should go in expecting it. Some people accept them only with the labour terms stated in writing and a clear statement that a failure means a second chargeable job.
It is also worth inspecting a used unit more carefully than a new one before fitting, and describing what you found. A note that the housing was cracked or the connector was damaged is worth much more before installation than after.
The customer supplied a part and also wants you to diagnose the fault
Separate the two jobs and price them separately. A diagnosis is a service that stands on its own, and the guide in this cluster on pricing a diagnostic fee covers how. Doing the diagnosis first and the fitting second, as two decisions, avoids the situation where a diagnosis contradicts a part that has already been bought.
If the diagnosis says the supplied part is not the answer, that conversation is much easier when the diagnosis was a paid piece of work rather than something thrown in.
It is a long standing customer asking as a favour
Favours are how policies get eroded, and also how good relationships work, so the practical approach is to keep the terms and be flexible on the price rather than the reverse. Waiving the labour terms is what creates the argument later; discounting the labour does not.
If you do make an exception, say out loud that it is one. An unmentioned exception becomes the new expectation.
The part is fine but it is the wrong one for the vehicle
This is the most common outcome and it is worth handling as a separate case from a faulty part, because nothing is broken. The visit still cost you the trip and possibly some stripping down, and the policy usually covers that as chargeable time.
The most useful thing you can do here is identify the correct part precisely, with the number, so the customer can order it once rather than twice. That single act turns a wasted visit into a reason they call you again.
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.
You have already fitted a supplied part with no written terms
Deal with the immediate situation on its merits rather than on principle, and treat it as the reason to write the policy this week. Arguing a term the customer never saw is a poor position even where it would otherwise be reasonable.
Where possible, put whatever you agree now in writing, including anything you have decided to absorb, so at least the second half of the job has a record.
The customer is refusing to pay the labour on a failed supplied part
Go back to what was agreed and what is recorded. If the estimate they approved said labour is payable regardless, say so once, calmly, and in writing. If nothing was recorded, decide quickly whether this is worth the relationship and the review, because a disputed labour charge on a part you did not supply is rarely worth a long fight.
Whatever you decide, write the outcome down and change the paperwork so the same conversation cannot happen the same way twice.
You are being asked to fit a part you believe is unsafe
Decline, and say plainly why in a way that can be read back later. This is one of the few places where the answer is not a negotiation, and the reason to write it down is not the argument, it is that a note describing what you saw and what you refused is the record that matters most if anything happens afterwards.
Charging for the visit is normal where the policy said the visit is chargeable, and it is much easier to charge when the policy said so in advance.
Questions
Should a mechanic fit parts the customer bought?
It is a business decision rather than a rule, and all three common positions are defensible: refuse, accept under written terms, or accept selectively. What matters more than which you choose is that you choose once, write it in plain language, and show it to the customer before the job is booked.
Who is responsible if a customer supplied part fails?
That depends on what the two of you agreed and on the rules where you work, which differ by state, so this page cannot answer it for your situation. What written policies usually record is that the part warranty sits between the customer and whoever sold it, while the fitter stands behind the workmanship. Getting that agreed in advance is the point of writing it down.
Can I charge labour twice if a supplied part fails?
Policies commonly state that a repeat visit is chargeable, because the work is genuinely being done again. Whether that holds in a particular dispute depends on what the customer agreed to and where you are, so it belongs in the estimate they approved rather than being raised for the first time on the second visit.
Should I charge more labour on supplied part jobs?
Some do, on the reasoning that the parts margin which normally covers collection, cash tied up and warranty risk is absent. Others keep the same rate and manage the risk with terms instead. This page does not recommend a figure or a percentage, because no sourced comparison of what anybody charges exists here to base one on.
What should I do if the part arrives after I do?
Treat it as a visit that happened, which is why the policy usually covers a chargeable trip. Asking the customer to confirm the part is physically in their hands before you set off prevents most of these, and it is a reasonable thing to insist on for mobile work in a way it would not be for a shop.