Some E-ZPass customers of the Maine Turnpike Authority may have been overcharged because of a damaged sensor at the Kennebunk toll plaza that officials allowed to stay open for three months.

Turnpike spokeswoman Erin Courtney said Wednesday that turnpike officials first learned of the potential overcharges last week when a customer called to report extra charges to an E-ZPass account.

Courtney said turnpike staff concluded that the problem was isolated to one lane at the Kennebunk toll exchange and stemmed from inaccurate readings by a sensor in the road leading to the E-ZPass toll booth on the southbound entrance ramp to the turnpike. The sensor was damaged by a snow plow but the lane remained open from January through March while repairs were made to the only other active E-ZPass lane at the southbound entrance ramp.

All E-ZPass customers have sensors mounted to the windshields of their cars to track when they pass through a toll plaza, but they also drive over a pavement sensor at each toll plaza that counts the number of axles on the vehicle. If a standard two-axle vehicle, for instance, is hauling a two-axle trailer, it is charged a higher rate.

The charge for a two-axle vehicle entering the turnpike at that exchange is $1 for both cash and E-ZPass customers. The charge for a vehicle hauling a trailer is $1.50, according to the toll calculator on the Maine Turnpike’s website.

The damaged sensor at the Kennebunk toll plaza apparently registered one or two extra axles on some vehicles, Courtney said.

Courtney said turnpike officials were aware the sensor was damaged but had to keep it open because there were no other E-ZPass lanes available. She said it’s not uncommon for snowplows to damage sensors, but they are usually fixed immediately. She said the turnpike has never previously had any similar issues with these sensors, which are manufactured by International Road Dynamics of Saskatchewan.

An official with the company, which produces a number of items, including traffic data and traffic safety products, could not immediately answer questions about whether their sensors have had similar problems.

Courtney said turnpike officials have no way of knowing how many E-ZPass accounts were affected because they cannot determine whether vehicles were overcharged by mistake or were actually pulling a trailer. She said customers should check their accounts for the months of January, February and March to see whether they were overcharged. Anyone with inaccurate charges should contact the turnpike customer service center at (888) 682-7277.

“Based on the low level of calls we have had on this issue over the past three months, we don’t have any reason to suspect that it was causing inaccurate reads more than infrequently,” Courtney said.

At Kennebunk, motorists are charged only when they enter the turnpike, not when they exit. The damaged sensor affected only vehicles headed south.

The Exit 25 toll is among the least frequently used of the turnpike’s 17 toll exchanges. Slightly fewer than 2 million vehicles passed through that toll exchange in 2013, accounting for about $1.8 million, or 2.5 percent of total turnpike revenue. Only the Rand Road exit in Westbrook took in less money last year.

Turnpike officials said they could not provide a figure for the number of E-ZPass trips through the Kennebunk toll exchange. There are about 200,000 E-Z Pass customers in Maine.

Staff Writer Eric Russell can be contacted at 791-6344 or:

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Twitter: @PPHEricRussell