Why don't you just replace the part the fault code names?

Because a fault code shows which system has reported a problem, not always which part has failed or why. Fitting parts on a guess can cost you money without fixing the car, so we test to prove the cause first.

Will you do any work without asking me?

No. You get the evidence and a written price first. Nothing is repaired, and no further testing is done, until you have agreed to it, and repairs need your written approval.

How will I know what is happening with my car?

We give you a promised time for your next update at each stage, so you are not left chasing us.

What if you find something else during the repair?

We stop and tell you. You get the evidence, with photographs, and a written price, and the extra work is done only if you approve it in writing.

How much does diagnosis cost?

Request a quote with your registration and a description of the fault, and we will tell you the price before any work starts.

How we diagnose

A fault code is not a repair: our diagnostic process

A fault code is not a repair, so we test the car and prove the cause, with photographs as evidence, before we recommend any work. Nothing is repaired until you have a written price and have given your written approval, and afterwards we check the repair has worked, with a test drive where appropriate.

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Last updated 12 November 2026

Why is a fault code not a repair?

When a car’s systems detect a problem, they store a fault code. The code shows which system has reported the problem. It does not prove which part has failed or why.

Fitting parts on the strength of a code alone can cost you money without fixing the car. So we use the code as a starting point, then test until the cause is proven.

What are the steps?

You tell us about the fault. When it happens, how often, what the car was doing at the time and any warning messages you have seen. You receive: a promised time for your first update.
We test. We read the stored fault codes, then test the system they point to. Where it is safe and useful, we drive the car to see the fault for ourselves. You receive: updates at the times we promise. If the cause needs more testing than we agreed, we tell you what and why, and we go further only when you agree.
We prove the cause. We find which part or connection has failed and record what we find. You receive: the evidence, with photographs, and a plain explanation of what we found.
You decide. We recommend the repair. You receive: a written price. Nothing is repaired until you give us your written approval, so there is no surprise invoice.
We fix the fault. We carry out the repair you approved. You receive: word from us if we find anything else. Any extra work comes with its own evidence and written price, and waits for your written approval.
We verify the repair. We test the system again and, where appropriate, test drive the car to check the fault has gone. You receive: your car back with the fault checked, and an invoice that matches the work you approved.

What if the fault comes and goes?

Intermittent faults do not always show up on the day. The more you can tell us about when and how the fault appears, the better our chance of seeing it. While we work on it, you still get a promised time for each update, and no further work is done without your agreement.

Where do I start?

For a warning light, misfire, running fault or electrical problem, see car diagnostics.

If you want a set first assessment, Diagnose & Plan ends with a written plan and price, so you know the next step before you commit to a repair.

For a diesel particulate filter, AdBlue or EGR concern, see the DPF and emissions health check. For an automatic or dual-clutch gearbox, see the transmission condition check.

Does the same process apply to servicing?

Yes. If we find something during a service or MOT that needs attention, you get the evidence and a written price, and nothing extra is done without your written approval.

How do I book?

Use the enquiry form on this page. Choose “Diagnostics or warning light”, give your registration and describe the fault in the message box.

Our promise on every job

Written approval before any extra work
Photographic proof of what we find
A promised update time
No surprise invoice
A test drive and verification where appropriate

Our service centre

Where will my car be looked after?

At our centre at 57 Great North Road, Welwyn Garden City, built so that waiting for your car is not wasted time.

Seven ramps and an on-site MOT bay
Three customer waiting areas
A co-working space with Wi-Fi
A multi-faith prayer room and quiet room
Lavazza coffee
Customer parking

Questions people ask us

Why don't you just replace the part the fault code names?: Because a fault code shows which system has reported a problem, not always which part has failed or why. Fitting parts on a guess can cost you money without fixing the car, so we test to prove the cause first.
Will you do any work without asking me?: No. You get the evidence and a written price first. Nothing is repaired, and no further testing is done, until you have agreed to it, and repairs need your written approval.
How will I know what is happening with my car?: We give you a promised time for your next update at each stage, so you are not left chasing us.
What if you find something else during the repair?: We stop and tell you. You get the evidence, with photographs, and a written price, and the extra work is done only if you approve it in writing.
How much does diagnosis cost?: Request a quote with your registration and a description of the fault, and we will tell you the price before any work starts.