mazda headlights...

[Sidewaze Samm]

In my entire motoring life over four decades, I think I have needed to change a headlight bulb on my cars on two occasions. But I'm puzzled why I've needed to change bulbs in my daughter's Mazda four times in under two years.
Is this just sheer bad luck? As Churchill may have said "once is happenstance, twice is coincidence, thrice is enemy action and four times is a crap car" .
Both sides have failed, but only ever one bulb at a time, and they include standard OEM from Halfords as well as premium Phillips and Osram.
Any ideas?

[pvr]

What is the output at the lights with the engine running? Could the voltage be too high given by the alternator?

[Sidewaze Samm]

What is the output at the lights with the engine running? Could the voltage be too high given by the alternator?

I don't have the tools or knowhow, but I know a man who does....
Cheers

[Sidewaze Samm]

Alternator output tested and is fine.
Suggested slight corrosion on terminals could be to blame?

[BeeEmm]

This happened to my wife's mark1 Ford Focus. We change a bulb 3 times in 3 years until we were told that they should be replaced in pairs (no, not by Halfords) even though only one bulb had blown. We did this and have not replaced the bulbs after 3 years so far.

[buzyg]

It's certainly not a Mazda thing that I have noticed. We have had Several models over many years currently have two sat on the drive. Only one bulb failure that I recall, on MrsG's current Mx5 Annoying niggle though, hope you find a simple solution. Certainly worth changing both at once, though never done that myself, it can't hurt. Always changed them my self too. Wouldn't use Harfords to fit a Tea bag in a pot.

[Sidewaze Samm]

Wouldn't use Halfords to fit a Tea bag in a pot.

[Barty]

Had the exact same thing on my first 159; changed a bulb every 3-4 months, but only one at a time. Once given the same advice to swap both at the same time, and suddenly no issues anymore Haven’t quite figured out why, but it worked for me

[Chris_D]

I can suggest that there is probably a voltage regulation circuit located somewhere between the alternator and headlight wiring that calibrates a load current for both headlights at the same time and not individually.
Ever notice how some old cars or bikes lights brighten/dim when blipping the throttle? Because they’re working on a fixed voltage supply.
On modern cars the voltages coming from the alternator fluctuate under load/supply and become somewhat volatile, then surge protection becomes necessary via a voltage regulator circuit and preventing over-voltage to various sensitive circuits and components. EV’s have well-developed VRC’s for obvious reasons.
There will be a higher current draw required for the degrading bulb and the VRC will respond within certain parameters and increasing voltage slightly to the headlight circuit. The degraded bulb will eventually fail but the circuit may be conditioned for a higher than normal voltage supply meaning a new bulb will then last longer than the 2nd original bulb which will also be subject to an accelerated failure mode before also eventually failing before the VRC recalibrates. Hence, why it’s also suggested to replace bulbs in pairs as they are both likely to withstand over-voltage conditions better than a pair of dissimilar age/use.

[Barty]

Every day’s a school day! That sounds like a very logical explanation, thanks Chris_D