A D.C. ambulance rushing a gunshot victim to a hospital Wednesday had to pull over to avoid engine failure that fire officials blamed on an emissions system required by the Environmental Protection Agency.

Paramedics were performing CPR on Nathanial McRae, 34, when an indicator light signaled that engine failure was imminent. They waited seven minutes for another ambulance to arrive, and McRae was later declared dead at Howard University Hospital. Deputy Fire Chief John A. Donnelly said the ambulance delay did not impede McRae’s care.

The ambulance pulled over on Suitland Parkway near Route 295. Police officers had shot McRae, a carjacking suspect, during a gun battle in the 800 block of Barnaby Street NE.

A person who answered the phone at the victim’s house said, “We’re sad, and we’re grief-stricken,” but had no further comment.

The diesel engines at issue are designed to cut power if exhaust filters are not kept clean, a process that requires the vehicles be taken out of service for up to an hour every few days to burn off accumulated soot and allow the filtering system to perform well. The process is known as “regenerating.”

Types of first responders

A warning light is supposed to flash and give the driver enough time to complete an emergency run before taking a scheduled break. Donnelly said that didn’t happen Wednesday; instead, a more severe indicator came on warning of imminent failure.

“That is not supposed to happen,” the deputy chief said, noting that he was awaiting results of a diagnostic test to determine whether the breakdown was the result of a clogged filter or some other problem.

Either way, the revelation that a system required by the EPA could force an ambulance off the road during an emergency has raised questions about whether fire apparatus should be exempt from the rules.

It also left another blemish on the D.C. fire department, which has been plagued with problems, including delays in response times. Even with the burden of the EPA requirements, Wednesday’s incident revived attention on whether fire department officials are properly managing their fleet of trucks.

Donnelly, who is in charge of the department’s fleet, said 32 of the District’s 94 ambulances require regeneration. He said Wednesday’s incident was the first such breakdown.

The emissions issue, however, appears to be one departments are grappling with nationwide.

“I know they’re trying to reduce pollution emissions, but I don’t know if they contemplated all the dangers,” said Thomas R. Wood, the chief of fire rescue services in Boca Raton, Fla. “Fire doesn’t take a timeout to let firefighters regroup and regenerate.”

Last year, the EPA, facing criticism from fire chiefs and trade groups, allowed for exceptions so that emergency vehicles “would no longer face power disruptions.”

But Harold Boer, head of the Fire Apparatus Manufacturing Association, said the waiver does not fully exempt emergency vehicles and instead allows them to be retrofitted so there is more time between regeneration stops. Boer, who is also president of the fire truck builder Rosenbauer, said few cities request the work because it does not eliminate the problem. He said a request to the EPA for a blanket exemption for all emergency vehicles has been denied.

Donnelly said the D.C. fire department, through a manufacturer, obtained one EPA waiver for a new foam truck that is dispatched to the White House for presidential helicopter landings and takeoffs. But he described it as “quite an arduous process” and said it was granted because the unique design of the foam truck could not accommodate the filter system.

In response to an interview request, an EPA spokesman sent the following statement: “A properly working and maintained vehicle should not shut down without adequate warning, and pollution control equipment does not have this general impact — as evidenced by the millions of vehicles on the road that have been operating with this technology for years.”

The new filtering systems are required on all diesel engines built after 2007. Regeneration, however, is more challenging for emergency vehicles and other trucks that do a lot of stop-and-go city driving. In commercial trucks, by comparison,the filters can be cleaned automatically by driving for about 20 minutes at a sustained speed above 45 mph. Ambulances in big citiesrarely travel that fast for that long. Idling causes the filters to dirty quickly, and ambulances idle for hours at emergency scenes and hospitals, several fire officials said.

Wood, the Boca Raton chief, said his department puts each engine and medic unit into a more-involved “forced regeneration” that keeps the filters clean for about a month rather than the four or five days they otherwise can last before more limited service is required. The plan works, he said, but at the cost of pulling vehicles out of service for lengthy periods.

“No matter what we do, including using different maintenance cycles, the vehicles are out of service more than necessary,” said Neil Blackington, the deputy superintendent of Boston’s emergency medical service.