The report by Germany's Federal Motor Transport Authority (KBA) comes after France banned the sale of most of Daimler's new model Mercedes cars using the older refrigerant, saying it did not meet new European environmental standards.
Ever since Daimler warned last year that the new chemical known as HFO-1234yf poses a greater fire hazard than previously believed, a dispute has raged in the auto industry over a component that few car owners regularly think about - the refrigerant in their air conditioners.
The KBA recommended further examination of the new refrigerant - whose global supply is controlled by Honeywell and its partner DuPont - to limit risks as much as possible.
At stake is not just carmakers' reputation for safety or the billions of euros that Honeywell could reap in sales, but the definition of 'acceptable risk', a notion that few carmakers relish addressing in public.
Were fears over the Honeywell product to spread outside Germany, justified or not, the auto industry risks being confronted again with the need for a costly search for a new chemical that meets an EU directive on air-conditioning refrigerants in cars.
No serious risk cited
KBA carried out tests using three different levels of severity and concluded that while the new substance was more hazardous than the old, it did not comprise a serious danger.
"No sufficient proof was found with the cars tested that would have hardened the suspicion of a serious danger as defined by the product safety law," the KBA said in its report.
Its most severe test, Stage 3, showed 1234yf was more dangerous than refrigerant 134a, which is currently used, but conceded that it was not entirely clear what conditions were necessary for the Honeywell product to become a serious hazard.
"Due to the comparisons with the previous refrigerant 134a in Stage 3, one can ascertain that the safety level of cars tends to deteriorate when 1234yf is used," it explained.
Report sent to EU
The recommendation was made in an official report sent to the European Union, which has intervened to mitigate the Franco-German dispute, and was obtained by Reuters on Thursday (8 August). The KBA later confirmed the report.
Had the KBA determined a material risk existed as defined by Germany's product safety laws, it could have triggered a recall of all cars on the road currently using 1234yf.
Nevertheless the agency had determined that in particularly severe crashes, one of the four models tested had burst into flames and emitted a considerable amount of toxic hydrogen fluoride (HF) gas in the process.
"Non-negligible" amounts of HF were also detected in two other test crashes, even if the refrigerant itself did not appear to ignite, according to the agency.
A spokesman for the KBA declined to say which models were affected in the most severe stage, since these round of tests surpassed what was necessary to ensure product safety.
The four models tested were a Mercedes B-Class, a Hyundai i30, Subaru Impreza and Opel Mokka.
A comprehensive final report is due for mid-September.
Global carmakers were so concerned about the KBA report that they had successfully convinced the EU it needed to intervene and review the findings in order to guarantee impartiality.
Daimler said the KBA report confirmed the automaker's decision to continue using older refrigerants while the firm develops systems that use non-flammable carbon dioxide as a coolant.