O'ahu Transit Services pulled 10 hybrid electric-diesel buses out of service this week following an engine fire in one of the buses on Tuesday, the third fire in the past year on the fuel-efficient buses.

City spokesman Bill Brennan said OTS officials decided to pull all 10 of the new buses off the road as of Wednesday as a precaution.

The previous fires occurred in December and January, he said, and those blazes were traced to a clogged particulate filter. Brennan said representatives of the bus and engine manufacturers will investigate what caused the fires.

In January, after the second fire, bus mechanics began a new maintenance procedure that included nightly inspection of the particulate filter.

There was no clog in the filter of the bus that burned this week. City officials are still investigating what might be causing the overheating problem, Brennan said.

Brennan said transit officials hope the fix will be simple and that the buses will be back on the road soon.

The buses were bought last year under then-Mayor Jeremy Harris. The 75-passenger vehicles cost $749,000 each and were touted as cleaner, quieter and more fuel-efficient than conventional diesel buses.

They were first running on a Kaka'ako-to-Waikiki route but Mayor Mufi Hannemann reassigned them to a new loop from Waipahu to the University of Hawai'i-Manoa because so few people were riding the in-town E-transit line.

Brennan said service will not be reduced by the change. "We've substituted other buses," he said, to make up for the 10 that are parked until the problem can be pinpointed.

Hannemann said the route was operating at about one-third of capacity and estimated that it had cost the city about $2 million to operate in the initial months of service.

Hannemann said the new buses also were disappointing because they do not climb hills efficiently, so couldn't handle routes such as the trans-Ko'olau trek to Windward O'ahu.

Reach Robbie Dingeman at rdingeman@honoluluadvertiser.com.