Dozens of Ford Explorer owners in the US have filed claims against the car maker amid growing complaints of suffering carbon monoxide poisoning symptoms from using the SUV. An investigation into the concerns is underway.

According to a Bloomberg report, 3,000 Ford Explorer users have reported they suspect exhaust gases have leaked into their vehicle and that they’ve experienced some of the symptoms of carbon monoxide poisoning, which include headache, dizziness, nausea, and loss of consciousness.

In one instance, Ford Explorer owner Bert Henriksen was told by his doctor that his carbon monoxide exposure was consistent with someone who had been in a house fire.

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When Ford investigated some of the claims, it found the Explorer exhaust system wasn’t welded properly, and gaps were left through which the exhaust gases could escape. It instructed dealerships to repair the systems, and Ford told CBS a repair fix that was made available in 2017 “effectively resolves that matter,” however a number of owners say the fix offered wasn’t good enough.

The first complaints about exhaust smells were made soon after the fifth-generation Explorer started production in 2011. A Ford employee was the first to note the issue, but Ford found that the circumstances in which the issue occurred took place outside “typical customer use.”

Ford spokesman Mike Levine told Bloomberg that the company’s testing showed the SUVs were safe, saying: “Ford's investigation has not found carbon monoxide levels that exceed what people are exposed to every day.”

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At least 50 customers have taken legal action against the company over the exhaust leaks. Ford has also bought back many of the vehicles from unhappy customers as “goodwill gestures,” with federal records showing it had bought around 100 by 2016.

The National Highway Traffic Safety Administration (NHTSA) has actually been investigating the Explorer problems since 2016. Its website has numerous complaints about it. Should it decide to enforce a recall, Ford would stand to lose millions, as the fifth generation Explorer has been on the market for eight years.

However, proving carbon monoxide exposure is difficult, as doctors don’t regularly check patients for it, and the US doesn’t have a regulatory standard for how exposure is a health risk.

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