diff --git a/HISTORY.rst b/HISTORY.rst
index 4b172830..07021307 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -18,7 +18,7 @@ PlatformIO Core 5
 - Fixed an issue when the ``$PROJECT_DIR`` variable was not properly replaced in the `debug_server <https://docs.platformio.org/page/projectconf/section_env_debug.html#debug-server>`__ option (`issue #4086 <https://github.com/platformio/platformio-core/issues/4086>`_)
 - Fixed an issue when `PIO Remote <https://docs.platformio.org/page/plus/pio-remote.html>`__ device monitor crashes on the first keypress (`issue #3832 <https://github.com/platformio/platformio-core/issues/3832>`_)
 - Fixed "Do not know how to make File target 'debug'" issue when debugging project using `CLion IDE <https://docs.platformio.org/page/integration/ide/clion.html>`__ (`pull #4089 <https://github.com/platformio/platformio-core/issues/4089>`_)
-
+- Fixed "UnicodeEncodeError" when a build output contains non-ASCII characters (`issue #3971 <https://github.com/platformio/platformio-core/issues/3971>`_)
 
 5.2.2 (2021-10-20)
 ~~~~~~~~~~~~~~~~~~
diff --git a/platformio/platform/_run.py b/platformio/platform/_run.py
index 09642689..00588c5a 100644
--- a/platformio/platform/_run.py
+++ b/platformio/platform/_run.py
@@ -110,6 +110,8 @@ class PlatformRunMixin(object):
             args.append("%s=%s" % (key.upper(), self.encode_scons_arg(value)))
 
         proc.copy_pythonpath_to_osenv()
+        # force SCons output to Unicode
+        os.environ["PYTHONIOENCODING"] = "utf-8"
 
         if targets and "menuconfig" in targets:
             return proc.exec_command(
diff --git a/platformio/proc.py b/platformio/proc.py
index 0484cc42..f041d61c 100644
--- a/platformio/proc.py
+++ b/platformio/proc.py
@@ -30,7 +30,9 @@ from platformio.compat import (
 class AsyncPipeBase(object):
     def __init__(self):
         self._fd_read, self._fd_write = os.pipe()
-        self._pipe_reader = os.fdopen(self._fd_read, errors="backslashreplace")
+        self._pipe_reader = os.fdopen(
+            self._fd_read, encoding="utf-8", errors="backslashreplace"
+        )
         self._buffer = ""
         self._thread = Thread(target=self.run)
         self._thread.start()