* Prosecutor says investigating two employees of Bosch
* Says probe relates to Grand Cherokee, Dodge Ram models
* Found signs of reduced effectiveness of emissions controls
* Bosch already investigated over VW, Porsche, Mercedes vehicles (Adds Bosch comment)
FRANKFURT, Jan 31 (Reuters) - The Stuttgart prosecutor's office said on Wednesday two employees of auto supplier Robert Bosch were being investigated on suspicion of aiding fraud, as part of a wider probe into Chrysler diesel emissions in the United States.
The Chrysler Grand Cherokee 3.0 L and the Chrysler Dodge Ram 1500, which have been on sale in the United States since 2014, had shown signs of reduced effectiveness of emissions control systems without technical justification, the office said.
The U.S. Environmental Protection Agency (EPA) in January 2017 accused Fiat Chrysler Automobiles of using hidden software to allow excessive diesel emissions to go undetected, leaving FCA facing a maximum fine of around $4.6 billion.
The Stuttgart prosecutor's office is already investigating Bosch for its role in designing engine management systems for Volkswagen, Porsche and Mercedes-Benz vehicles.
In September 2015, Volkswagen admitted to using engine management systems to cheat emissions tests by using software to detect testing procedures in order to adjust filtering systems to mask higher pollution levels.
In an emailed statement on Wednesday, Bosch said: "It is well known that the accusations of manipulation of diesel software are the subject of a preliminary legal proceeding and civil lawsuit also involving Bosch."
Bosch said it has supported the ongoing investigations and has been fully co-operating with the responsible authorities, and declined to comment further. (Reporting by Edward Taylor; Additional reporting by Jan Schwartz; Editing by Maria Sheahan and David Evans)