Up to 170,000 Toyotas on Australian roads including the top-selling HiLux utility have a design flaw that can force the engine to lose power without warning and life-saving safety systems to shut down.

But Toyota Australia, which markets the HiLux as ‘unbreakable’, says the air inlet fault is not a safety issue and it won’t issue a recall.

Instead it has requested Toyota Japan come up with a fix, but it can’t say when that fix will be introduced or if it will be retrofitted to about 170,000 2.8-litre diesel engines already in service in Australia in the HiLux, which was the No.1 seller in Australia in 2016 and 2017, the Fortuner medium SUV and Prado large SUV.

The design fault allows micro-sized dust particles to leak past the air filter and contaminate the readings of what’s called the mass air flow sensor (or MAF), which determines the engine’s correct fuel-air ratio.

The engine then goes into limp mode, turns off stability and traction control, and instructs the driver to go to a Toyota dealer.

The fault is present in vehicles built from 2015 onwards.

The discovery of the fault calls into question some of Toyota’s marketing claims.

“As the vehicle is subject to reduced engine power (‘limp mode’) and can continue to be operated safely, it is not a safety-related item that would require a recall,” a Toyota Australia spokesperson said.

“We do however encourage customers to contact their nearest dealer as soon as possible if this issue does present itself.”

While Toyota Australia issued service bulletins about the issue to dealers as far back as March 2017, it is only publicly acknowledging it now because a $64,000 HiLux Rugged X press vehicle went into ‘limp mode’ four times while being tested by reviewers on behalf of an automotive website.

The impacts were dramatic with the vehicle slowing suddenly and significantly, including one occasion when on the wrong side of the road overtaking another vehicle.

The spokesperson said the safety systems shut down in limp mode to ensure they do not misbehave because of the incorrect data being developed by the MAF. Stability control contains vehicle slides, while traction control subdues wheelspin.

“As the MAF sensor affects engine performance and is one of the vehicle systems that is used to control the vehicle in such instances, the vehicle takes a conservative approach and disables the safety systems entirely instead of making an inaccurate judgement using incorrect or incomplete data,” the spokesperson stated.

Toyota Australia claims the issue only emerges in extremely dusty conditions, but those are exactly the kinds of conditions many HiLux, Fortuner and Prado owners would find themselves in, as all three vehicles are sold as off-roaders.

Toyota has recommended vehicles used in those conditions have their air filters cleaned more regularly and the MAF be cleaned with compressed air.

But the real solution will be the air inlet redesign, although Toyota Australia says it doesn’t know when it will become available or whether it will be available to existing owners.

“At this stage exact timing is not known, as any change to the intake system will affect emissions and homologation and therefore will require extensive re-testing,” the spokesperson said.

“At this stage we believe it will not make it in time for the next technical change, but we will endeavour to expedite this so it is implemented at the earliest production opportunity.

“We have requested that any changes to the air intake or other related systems be made available also to vehicles already in the field. However, as the design is not yet complete, it is too early to say whether this will be possible.”