encode means characters to binary. What you want when reading a file is binary to characters → decode. But really this entire process is way too manual, simply do this:

with open('keys.json', encoding='utf-8') as fh:
    data = json.load(fh)

print(data)

with handles the correct opening and closing of the file, the encoding argument to open ensures the file is read using the correct encoding, and the load call reads directly from the file handle instead of storing a copy of the file contents in memory first.

If this still outputs invalid characters, it means your source encoding isn't UTF-8 or your console/terminal doesn't handle UTF-8.

You can open with codecs:

import json
import codecs

json.load(codecs.open('sample.json', 'r', 'utf-8-sig'))

or decode with utf-8-sig yourself and pass to loads:

json.loads(open('sample.json').read().decode('utf-8-sig'))

Have you tried:

json.loads(line.decode("utf-8"))

Similar question asked here: UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2

Edit: If the above does not work,

json.loads(line.decode("utf-8","ignore"))

will.

You are encoding to JSON then decoding again. SImply not encode, remove the second line:

decodedjson = data.decode('utf-8')
loadedjson = json.loads(decodedjson)

If you are using Python 3.6 or newer, you don't actually have to decode from UTF-8, as the json.loads() function knows how to deal with UTF-encoded JSON data directly. The same applies to Python 2:

loadedjson = json.loads(data)

Demo using Python 3.7:

>>> data = b'{"Current": 117.42609405517578, "Accelerometer": -5.394751071929932, "SpeedometerKPH": 67.12493133544922, "Ammeter": 117.3575210571289, "Amp": 117.35590362548828, "Acceleration": -0.03285316377878189, "TractiveEffort": -5.394751071929932, "Effort": 48.72163772583008, "RawTargetDistance": 3993.927734375, "TargetDistanceBar": 0.9777777791023254, "TargetDistanceDigits100": -1.0, "TargetDistanceDigits1000": -1.0}'
>>> loadedjson = json.loads(data)
>>> loadedjson['Ammeter']
117.3575210571289

What you get is valid UTF-8 JSON. It's just written as pure ASCII using escape codes for non-ASCII characters, which as a subset of UTF-8 is also valid UTF-8. Read it back in with json.load and it will be the original string. If you want the actual Unicode characters encoded as UTF-8 instead of escape codes when written to the file, use json.dump with the ensure_ascii=False parameter, and make sure to open the file with encoding='utf8':

with open("test.json", "w", encoding='utf8') as jsonFile:
    json.dump(data, jsonFile, ensure_ascii=False)

This is in the documentation:

json.dump(obj, fp, *, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, sort_keys=False, **kw)
...
If ensure_ascii is true (the default), the output is guaranteed to have all incoming non-ASCII characters escaped. If ensure_ascii is false, these characters will be output as-is.

Specify the encoding as a part of open(). Here is a "round-trip demo":

>>> import json
>>> data = {
...     "title": "قالت وزارة الداخلية المصرية إن كمية من المتفجرات في سيارة كانت معدة لتنفيذ عملية إرهابية أدت إلى الانفجار الذي وقع وسط القاهرة وأودى بحياة نحو 20 شخصا."
... }
>>> with open("/tmp/utf16demo.json", "w", encoding="utf-16") as f:
...     json.dump(data, f)
>>> with open("/tmp/utf16demo.json", encoding="utf-16") as f:
...     newdata = json.load(f)
>>> next(iter(newdata.values())) == next(iter(data.values()))
True

As mentioned in the comments, just because the data is originally UTF-16 encoded does not need you mean to write it back to CSV in the same encoding. You are perfectly free to load and decode using UTF-16, but then write out using UTF-8.