Why doesn't an exhaust back box have one standard price?
Because what it actually needs depends on the car and on what's wrong with it. Sometimes only the back box itself (the rear silencer) has corroded through and the rest of the system is sound; other times the box is fine but a joint or bracket near it has failed, which is a different job. A universal-fit box for a common car is a different cost to a vehicle-specific box that only fits one model.
What actually fails on a back box?
Rust is the usual cause. Exhausts live underneath the car in the wet, and the back box sits lowest and coolest, so it's often where condensation collects and corrosion starts first from the inside out. Short journeys make this worse, because the exhaust never gets hot enough for long enough to drive the moisture back out as steam.
How do you know if yours has gone?
The clearest sign is noise — a deeper, louder, more "rattly" sound from the back of the car, especially noticeable on start-up or under acceleration. Sometimes there's a visible hole or a patch of surface rust that's clearly thinner than the metal around it. A garage can usually confirm it in a couple of minutes on a ramp, where the underside is actually visible.
Is a noisy exhaust an MOT problem?
It can be, depending on what's actually wrong and how bad it is. Our guide to the most common MOT failures goes into this in more detail, rather than repeating it here — it's worth a read if you're due a test soon and you've noticed a change in noise.
Can you still drive it?
A blowing exhaust on its own is usually not an emergency, but it's not something to leave indefinitely either, because exhaust gases are meant to be carried away from the car, not leaking out underneath it. If you can smell exhaust fumes inside the cabin, that's a different and more urgent situation — worth getting checked straightaway rather than waiting for a service.
Why does ONEFIX quote it in writing before doing anything?
Because the actual fix only becomes clear once the car's up on a ramp and the full system can be seen and photographed. ONEFIX quotes the price in writing before any work starts, and nothing happens to the car without that quote being approved — so you're never agreeing to a job based on a guess over the phone.
Does the rest of the exhaust need replacing too?
Not necessarily. An exhaust is usually made up of several sections joined together — the front pipe near the engine, a middle section, and the back box at the rear — and they don't all corrode at the same rate. The back box often goes first simply because it sits lowest and holds the most moisture, which is why a repair can sometimes be limited to just that one section rather than the whole system.
What does the health check actually show you?
Every service includes a written digital vehicle health check with photographs, and the exhaust is one of the things that gets checked from underneath. If there's early corrosion starting, you'll see a photo of it, with time to plan for the job before it turns into something noisier or more urgent.
The honest bottom line
There's no fixed figure for a back box because the job genuinely varies car to car. What's consistent is the process: it gets looked at properly, photographed, and quoted in writing before anything is touched — so you're deciding with the real picture in front of you, not a guess. If your exhaust's got louder recently, book it in and ask for it to be checked as part of the health check.