Arduino mega and SAE J2610 SCI-bus

Yes, it's safe. Don't forget the 1k pullup resistor.

Yes, exactly.

Did you insert the Arduino Mega between the PCM and the Autel tool?
Your code resembles of an UART repeater.
Serial.peek() does not remove data from buffer, so the loop keeps repeating something, which is not good.
The received bytes look gibberish to me.

We tried to spy on SCI-bus traffic like this before but never succeeded.

It's better to use a logic analyzer for this purpose.
I recommend Muse Lab's nanoDLA and the PulseView app.

Or, if you want to get on with it right now, I recommend connecting the Autel tool directly to the PCM, and use Arduino Mega's Serial1 RX pin and Serial2 RX pin to sniff on the RX/TX wires.
Again, only the RX pins are used, since they are input pins and don't interfere with SCI-bus.

void setup() {
   Serial.begin(115200);
   Serial1.begin(7812); // rx
   Serial2.begin(7812); // tx
}

void loop() {
   if (Serial1.available()) {
       uint8_t data = Serial1.read();
       Serial.print("RX: ");
       Serial.println(data, HEX);
   }

   if (Serial2.available()) {
       uint8_t data = Serial2.read();
       Serial.print("TX: ");
       Serial.println(data, HEX);
   }
}


Admin I would like to thank you for your help, this is working now.

This is not in any way what I have read about the SCI-BUS that the pull-up should be on the receiving end.
OBD-7 pin is 12V with nothing connected and signals to ground for messages.

I connected the Arduino RX pin with voltage divider.
OBD-7 > 10kΩ ---- 6.2kΩ -- GND
                         |
                         |
                       RX1

OBD-12 is 5v so I connected that straight to RX2
Let Autel show me AC HI-SIDE PRESSURE
GOT---
TX: 22
TX: 3
TX: 29
TX: 4E
RX: 62
RX: 27 <<<<
RX: 0
RX: 0
RX: 0
RX: 89

Second bit changes with psi. It should be multiplied by 2 according to DRB DB Reader.
AC HI-SIDE PRESSURE: J2190?; xmit: 22-03-29; sc: Engine; 0x80008646
UNIT (DFLT/MTRC): PSI/KPa
SLOPE: 2
OFFSET: 0

27 hex-to-int = 39
39*2 = 78 psi << that matches Autel

I then disconnected autel and moved OBD-12 pin to TX1 and then I could send messages and listen to response.

byte message[] = {0x22, 0x03, 0x29, 0x4E};
Serial1.write(message, sizeof(message));