Custom PIDs and expressions

A custom PID is a request you write yourself: which bytes to send, how to turn the answer into a number, and where the number goes. It is how you read a value no profile covers, test a request before it goes into a profile, or send one value to its own MQTT topic.

Add a custom PID

A custom PID row. New adds a row, Store writes the table to the device.

On Automate, under Custom PIDs, press New and click the new row to open it:

Field	What it is
Name	The name of the value, up to 31 characters. It becomes the key in the JSON, the MQTT payload and the Home Assistant entity.
Init	AT commands sent before this request, separated by ;, for example ATSH7E4; to address the battery module. Custom Initialisation above the rows is sent once before the whole table.
PID	The request, in hexadecimal, up to 9 characters: 010C for engine speed, 221E3B for a manufacturer value. An odd trailing digit tells WiCAN how many answer frames to wait for, as on an ELM327.
Expression	How the answer becomes a number, below.
Min Value, Max Value	Answers outside these limits are dropped.
Period(ms)	How often the request is sent, 100 ms or more.
Destination Type, Send_to	Default: the value is only part of the snapshot (HTTP, the grouped MQTT topic, Home Assistant). MQTT_Topic: every reading is also published as {"Name": value} to the Send_to topic. MQTT_WallBox: every reading is published to Send_to as a bare number with two decimals, 173.00.

Press Store and restart WiCAN. Delete removes a row.

Expressions

An expression is arithmetic over the bytes of the answer. The bytes are counted from the first byte after the CAN id, including the length byte the module puts first, so for the answer 7E8 04 41 0C 1A F8 to the request 010C:

Byte	B0	B1	B2	B3	B4
Value	04 (length)	41 (service)	0C (the PID)	1A	F8

Engine speed is the two data bytes divided by four: [B3:B4]/4, which gives 1726 rpm here. For an answer that spans several frames, the frames are joined in order, with the sequence byte that starts each frame counted like any other, which is why profiles for EV batteries index bytes well into the thirties.

Write	For
B0, B1, ...	a byte, unsigned (0 to 255)
S0, S1, ...	a byte, signed (-128 to 127)
[B3:B4]	several consecutive bytes as one big-endian number, up to 8 bytes; [S3:S4] the signed version
B5:3	bit 3 of byte 5 (0 or 1)
+ - * /	arithmetic, with decimals
& | ^ << >>	bitwise and, or, exclusive or, shifts
( )	grouping

Common patterns, with the request they belong to:

Value	Request	Expression
Engine speed, rpm	010C	[B3:B4]/4
Vehicle speed, km/h	010D	B3
Coolant temperature, degrees C	0105	B3-40
Fuel level, percent	012F	B3*100/255
A 16-bit value in tenths		[B9:B10]/10
A signed temperature		S16
A flag		B14:7

Test a request before trusting it

The quickest check is to ask the car the same request from an ELM327 app and compare: Car Scanner's log shows every request and answer, byte by byte. The values WiCAN decodes appear at /autopid_data after the restart.

The standard PIDs are decoded for you

The Standard PIDs section knows the formulas of the OBD-II standard, so for engine speed, speed, temperatures, fuel and the other standard values you need no expression: AutoPID. Custom PIDs are for everything else, and for a standard value that your car answers only from another module.