I do no manage to obtain using pdflatex bold font for Chapter Name, prefix and number when \usepackage[T1]{fontenc} is loaded. The rendering is changing due to this package and the command \bfseries has no effect.
with the T1 fontenc package:

without the T1 fontenc package:

I found following related topics on the forum but was not able to find a solution to my issue. I would be very greatful for your help.
Chapter Title adjusting with koma-script CM fonts with T1 fontenc Curious interaction between KOMA, fontenc and hyperref when using hidden glossary entries
Note: I want to use only KOMA-Script command because of the known incompatibilities between titlesec package and scrbook documentclass.
\documentclass[fontsize=12pt,a4paper, headings=twolinechapter]{scrbook}
\usepackage{xcolor}
\usepackage{graphicx}
\usepackage{lipsum} % just to generate text for the example
\usepackage[T1]{fontenc}
\usepackage[utf8, latin1]{inputenc}
% the chapter format:
\KOMAoption{chapterprefix}{true}
\renewcommand*{\chapterformat}{%
\raggedright%
\mbox{\chapappifchapterprefix{\nobreakspace}%
\scalebox{2.5}{\bfseries\color{gray}\thechapter\autodot}}\\}
\renewcommand*{\chapterheadendvskip}{%
\vspace{-12pt}\noindent{\setlength{\parskip}{0pt}\Huge\hrulefill\par}%
\vspace{\baselineskip}
}
\addtokomafont{chapter}{\raggedleft\Huge\bfseries}
\begin{document}
\chapter{Title of the First Chapter}
\section{Title of the Section}
\subsection{Title of the Subsection}
\lipsum[1-2]
\chapter{Title of the second Chapter}
\lipsum[1-2]
\end{document}


\usepackage{lmodern}. Unrelated, but important: giveinputencjust one option; eitherutf8orlatin1, according to your file's encoding. – egreg Oct 21 '14 at 18:10