This is a very simple script that will check for failed systemd units by parsing the output of systemctl(1).

check_systemd_failed

#!/usr/bin/env python3

# Copyright (c) 2019 Karol Babioch <karol@babioch.de>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# This is a very simple script that will check for failed systemd units by
# parsing the output of systemctl(1). It will exit with code 2 (critical),
# when there is any failed unit and with 0 (ok) if there is none. In case of
# failure (i.e. systemctl not available), it will exit with code 3 (unknown).

from subprocess import CalledProcessError, SubprocessError
import argparse
import logging
import subprocess
import sys

logger = logging.getLogger(__name__)
logger.addHandler(logging.StreamHandler())

EXIT_OK = 0
EXIT_WARNING = 1
EXIT_CRITICAL = 2
EXIT_UNKNOWN = 3

parser = argparse.ArgumentParser(description='This checks for failed systemd units')
parser.add_argument('--systemctl-path', dest='systemctl_path', help='Location of systemctl binary (/usr/bin/systemctl by default)', type=str, default='/usr/bin/systemctl')
parser.add_argument('-d', '--debug', dest='debug', help='Enable debugging mode', action='store_true')
args = parser.parse_args()

if args.debug:
    logger.setLevel(logging.DEBUG)

cmdline = [args.systemctl_path, "--failed", "--no-legend"]
logger.debug("Running: %s", cmdline)

try:
    output = subprocess.check_output(cmdline, encoding=sys.getdefaultencoding())
except (OSError, CalledProcessError, SubprocessError) as error:
    logger.debug("Exception: %s", error)
    print("Failed to invoke systemctl")
    sys.exit(EXIT_UNKNOWN)

logger.debug("Output: %s", repr(output))

if not output:
    print("No failed units")
    sys.exit(EXIT_OK)

# Append all found services to list
failed = []
for line in output.splitlines():
    failed.append(line.split()[0])

print("Failed units: {}".format(", ".join(failed)))
sys.exit(EXIT_CRITICAL)