Is reading the fault codes enough?

No. A fault code shows which system has reported a problem. It does not prove which part has failed, so we test to prove the cause before recommending a repair.

Can you diagnose a fault that comes and goes?

Intermittent faults are part of the work we do. Tell us when the fault appears, how often, and what the car was doing at the time.

Do you trace electrical faults?

Yes. Electrical fault tracing is done in-house at our Welwyn Garden City centre.

What do I get after the diagnosis?

The evidence for what we found and a written plan with a quote for the repair. You decide what happens next.

How much does diagnosis cost?

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

Car diagnostics

Warning lights and faults diagnosed properly

A fault code is not a repair, so we test the car to prove what is causing a warning light, misfire or electrical fault before recommending any work. You get the evidence and a written plan, nothing is repaired without your written approval, and we verify the repair afterwards.

Request a booking

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.

So we do not fit parts on the strength of a code. We test, prove the cause, get your written approval, then fix the fault and verify the repair.

Which faults do you diagnose?

Warning lights
Misfires
Running faults
Intermittent faults that come and go
Electrical faults, traced in-house

How do you diagnose a fault?

We listen. Tell us what you have noticed: when the fault appears, how often, and what the car was doing at the time.
We read the fault codes. They show us where to start.
We test. We test the system the code points to until the cause is proven.
We show you. You get the evidence and a written plan, with a quote for the repair.
You decide. Nothing is repaired without your written approval.
We repair and verify. After the repair, we check the result, with a test drive where appropriate.

What evidence will I see?

We record what we find so you can see it for yourself:

photos
saved scan results
technician notes

What happens if a fault is hard to find?

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. While we work on it, we give you a promised time for your next update, and no further work is done without your written approval.

What about DPF and emissions faults?

For warning lights linked to the diesel particulate filter, EGR or AdBlue systems, see our DPF and emissions health check. We repair emissions systems. We never remove or disable them.

Is there a set first assessment?

Yes. Diagnose & Plan is our initial assessment for a car with a fault. It ends with a written plan, so you know what the next step is before you commit to a repair.

How do I book?

Use the enquiry form. Choose “Diagnostics or warning light” 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

Is reading the fault codes enough?: No. A fault code shows which system has reported a problem. It does not prove which part has failed, so we test to prove the cause before recommending a repair.
Can you diagnose a fault that comes and goes?: Intermittent faults are part of the work we do. Tell us when the fault appears, how often, and what the car was doing at the time.
Do you trace electrical faults?: Yes. Electrical fault tracing is done in-house at our Welwyn Garden City centre.
What do I get after the diagnosis?: The evidence for what we found and a written plan with a quote for the repair. You decide what happens next.
How much does diagnosis cost?: Request a quote with your registration and a description of the fault, and we will tell you before any work starts.