(Adds detail on EPA review, possible fines, background on diesel; updates shares)
Jan 12 (Reuters) - The U.S. Environmental Protection Agency on Thursday will accuse Fiat Chrysler Automobiles NV of using software that allowed excess diesel emissions in just over 100,000 U.S. trucks and SUVs sold since 2014, two sources briefed on the matter said.
The EPA told the automaker it believes its auxiliary emissions control software allowed vehicles to generate excess pollution in violation of the law. Fiat Chrysler declined to comment.
A person briefed on the matter said Fiat Chrysler does not agree with the EPA's assessment. An automaker can use an auxiliary emissions control device in limited circumstances to protect the engine from damage, but it must be declared to regulators.
Fiat Chrysler's U.S.-listed shares and Milan-listed shares were each down 14 percent on the news.
The EPA will announce the findings at an 11 a.m. ET conference call. It comes amid rising scrutiny by EPA of automaker emissions after Volkswagen AG admitted to cheating diesel emissions tests in 580,000 U.S. vehicles.
The EPA has for months declined to certify Fiat Chrysler's 2017 diesel vehicles for sale in the United States, but the automaker has continued to sell 2016 diesel models.
In September 2015, EPA said it would review all U.S. diesel vehicles following an admission from Volkswagen that it installed software in cars allowing them to emit up to 40 times legally permissible level of pollution.
On Wednesday, VW agreed to pay $4.3 billion in criminal and civil fines and plead guilty to three felonies for misleading regulators and selling polluting vehicles.
The EPA has extensively investigated the vehicles and Fiat Chrysler has turned over significant documents as part of the probe, two people briefed on the matter said.
Fiat Chrysler could face fines of up to $37,500 per vehicle if it is proven that it violated emissions rules.
The probe covers Fiat Chrysler diesel trucks and SUVs from the 2014-2016 model years. (Reporting by David Shepardson; Editing by Nick Zieminski)