Hi Gerry!
Nice to have another ambitious follower 
The both of us uses an L9637D, which changes the K-line one wire signal to a two wired serial signal.
It´s connected directly to the Arduino serial port (PIN 0 & 1).
With that, we have full control about the communication, which you might not have with the ELM327 in between.
For example: If you deactivate header information (ATH0) you will not be able to communicate with another control unit.
The sketches can be found here:
My solution which communicates by Bluetooth
Scissor´s solution, which uses a SD card
Where do you got the information about the PID and module from? (Just interested 
)
81 20 F1 81 15
81 means: no Length information (single byte only)
20 is Target
F1 is sender (you)
81 means "start communication"
15 is the checksum
82 20 F1 21
82 some fomat I don´t know. KWP only has 80, 81, C0, & C1
20 Target
F1 Sender
21 Read Data by local indentifier
30 PID
E4 Checksum
Against your expectation, you are talking to the module 20. Not 21, that only means to read live data (Service ID).
Before talking to the device, you need to wake up the ECU by fast init (you will find in the code). Afterwards you will probably have to start a diagnostic session by 0x10 0x80.
I think there are all needed infos about Arduino, soldering, documentation and code in my GitHub.
Which kind of vehicle do you own? Sounds like a quad