Ford Motor Co. and the U.S. auto safety regulator are looking into reports of carbon monoxide in the passenger area of late model Ford Explorer sport-utility vehicles after receiving about 20 complaints and a lawsuit filed by a customer, reported The Wall Street Journal.

In 2012, Ford issued a "technical service bulletin" to its dealers suggesting ways to remedy an exhaust smell in the vehicle, but hasn't classified it as a safety issue or begun a recall.

Angela Sanchez-Knutson in a lawsuit filed last week in U.S. District Court for the Southern District of Florida, alleges a test of her 2013 Explorer shows that more than 100 parts per million of carbon monoxide can accumulate into the cabin when the auxiliary rear air-conditioning is on and the engine is working at higher revolutions. That is 10 times higher than ambient air levels of the gas. A level above 70 parts per million could begin to hurt healthy people, especially over longer exposures, according to the Consumer Product Safety Commission.

Her suit, which seeks class action status, asks the court to force Ford to recall and repair the vehicles, alleges breach of warranty, unfair trade practices violation and seeks unspecified damages.

Ford issued the service bulletin in December 2012 to dealers, giving them three different recommendations for addressing the problem. These service bulletins instruct dealers on how to fix a customer complaint, not safety issues, which are handled in the form of a recall.

Auto makers have recalled tens of millions of vehicles in 2014 as the light has been shown on the process at General Motors Co. for taking 11 years to recall 2.6 million vehicles that had an ignition switch flaw that could accidentally shut off air bags and power steering and brakes. That flaw is linked to at least 13 deaths and has put GM under scrutiny from lawmakers and the Justice Department.

National Highway Traffic Safety Administration said it "is aware of the complaints involving 2011-2014 Ford Explorers" and "is reviewing all available data and will take appropriate action as warranted." The safety regulator hasn't started a formal investigation.

The agency has collected 20 reports from Ford Explorer customers of exhaust fumes coming into the cabin of their vehicles. One report said a passenger had to go to the hospital after becoming "violently ill" for unknown reasons. In many of the reports, customers said they took the vehicle to a dealership and it was returned without repairs, documents show.

"The fixes that the technical service bulletin recommends were all done to this car and it didn't fix the exhaust smell in the car," said Michael Hersh, the attorney representing Ms. Sanchez-Knutson. "They still smell exhaust," he added.

Mr. Hersh said the dealer said there was no carbon monoxide flowing into the vehicle and that it wasn't a health risk. The attorney said he had the vehicle tested and that test found 100 parts per million of carbon monoxide in the interior of the vehicle.

Ford said in a statement on the court complaint, "we're currently reviewing the case and in the event that any action is required, we will address it promptly."