MoT history concerns

The Mk3.5 PRHT I’m looking to buy has some history of corrosion around the rear subframe. Should I be unduly concerned? These were classified as major.
Previous MoT - 47k 5.3.6 (a) (i). Excessive corrosion O/S rear structural inner seal/floor. Unsure how this was repaired but subsequently passed clean.
Current MoT this month. 47.5k 5.3.3 (b) (i). O/S and N/S rear suspension component (?) attachment bracket corroded and seriously weakened. Again repaired (how?) and then issued clean MoT
Car seems very tidy otherwise. Any guidance appreciated.

Get under it. i discounted about 7 before finding mine. Corrosion was 75% of the ones i discounted.

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There are plenty more out there without jumping to buy the first one.
As above, which ever one you do intend to purchase, get underneath it with a torch and look.
I also viewed a few before purchasing👍🏻

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“O/S rear structural inner seal/floor”

Would be sill/floor I’m guessing, a common place for these to rust.

See picture on mine that I treated. It’s under the car at the point the rear wheel arch meets the sill, behind the sill around the rear jacking point.


The other attachment brackets are I think these in the video…

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If it’s that bad walk away . Unless you can get it cheap. but the cost can be high to get her fixed. Look around as the winter is near so prices do drop .

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You have to look, really. Some MOT testers will go overboard on corrosion and some will ignore even really bad stuff. Bear in mind that the history is there forever though, so, even if the tester was trigger-happy with advisories, it will always worry future buyers that bother to check.

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If a sub frame is corroded, is that something that can be swapped out, rather that welding required if it’s body work?

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Completely agree - we have some very “advisory happy” MOT testers in the motorcycle world.
It can kill the value of a bike in later years because it looks unmaintained.

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Thanks all for your helpful replies. I tend to be prone to buyer’s remorse but on this occasion I’ve decided to complete the purchase. The specialist dealer has sent me a photograph of the affected area showing the previous welded repair and the new replacement bracket. All waxoyled so should get a couple of years’ protection. Fully serviced and new MoT is clean. Now to enjoy the car!

Thanks! Photo attached below :+1:

For me - i think i would of walked. The welding and size of plate BUT im proper happy you’ve got one and i hope you really enjoy it!!!


I just bought a 3.75 and was shocked by just how many of them were starting to show signs of serious corrosion.

Don’t fret too much about the subframe support brackets, they were the only thing showing anything like serious rust on mine so I just replaced them for 69 quid the pair. See link: NEW Mazda mx5 mk3 3.5 3.75 rear subframe brace bar pair 2006-2015 | eBay

Old vs new.


I’m not a fan of black underseal slapped on over everything. You don’t know what’s underneath. I just use clear Bilt-Hamber wax now.


Likewise. I saw many cars advertised as being waxoyled etc but I’d rather see the shell warts and all than a shiny black surface that could be concealing literally anything.

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Yes. Totally agree. Once the waxoyl needs replacing in 12-24 months (?), I plan to have it professionally cleaned off, underside fully inspected, any repairs done and then resealed with a recommended product. Maybe something clear rather than fault-concealing black gunk.

I would have it cleaned off ASAP so you can see what’s underneath.
By the look of it on the picture, there seems to be a lot of lumps that look like hidden corrosion.

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I do tend to err on the side of caution but I’m inclined to leave things be for a little while. The welded repair that ran close to the rear suspension bracket mount was done following the previous MoT some 9 months ago. There was little or no waxoyl in that area before the car had it’s current MoT so was inspected in a fairly “naked” condition. After the inspection (fail) both support brackets were replaced and the general area treated with Waxoyl. Some other components with “advisory” corrosion were wire brushed on treated too.
I use a local garage for servicing my other car and have a 10 year relationship. I’m going to have the NC in for a wheel alignment and I’ll get my usual guy to give me some feedback as to the condition of the underside in general. Fingers crossed it’s not a basket case or what’s really the point of an MoT? Car has only two previous owners in the Staffordshire area so I wouldn’t have thought road and climate conditions were not too bad by standards. Car being delivered Monday btw.

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