(Reuters) – German carmaker BMW ceased its role in an automotive industry research program investigating the safety of a potential new air conditioning refrigerant, disagreeing with the test methods employed.

BMW has now joined Daimler and Audi, according to a source familiar with the matter. BMW quit the group before it was due to issue its next report in the middle of February. Final findings are scheduled to be published in the second quarter of this year.

“Today we have ended our observer role in the working group,” a spokesman for BMW said on Wednesday, adding the company is continuing its own internal research into the issue.

“We do not want to say the test results are wrong, but we are not convinced the methods applied are sufficient to achieve a definitive conclusion that guarantees our high safety standards,” he said.

Flammability tests by Daimler last year sparked new concerns after years of research into the safety of the proposed new refrigerant, called HFO-1234yf, which European regulators want adopted by automakers to cut atmospheric emissions which have the potential to cause global warming.

Daimler’s initial doubts triggered the formation of the latest working group last year, the Cooperative Research Program (CRP) being conducted under the auspices of the international Society of Automotive Engineers (SAE), to further analyze the safety of HFO-1234yf.

The production of the refrigerant, which Daimler said releases a toxic gas when it burns, is being developed by Honeywell and its partner DuPont and could lead to a billion dollar monopoly starting in 2017 when a phase-out of current refrigerants is complete.

December last year, the research working group released a statement saying that none of the 12 other automakers involved other than Daimler “has provided information that would suggest a concern for the safe use (of HFO-1234yf) in their vehicles.

Daimler decided late in September to recall all 1,300 Mercedes cars using HFO-1234yf after simulated frontal car crashes repeatedly showed a leakage of the refrigerant-lubricant mix in the A/C system could spontaneously ignite when the surface of the turbocharged gasoline engine reached higher operating temperatures.