The question often arises: can you use an OBD2 splitter, specifically a 3-way splitter, to connect multiple OBD2 devices to your vehicle simultaneously? While physically possible, doing so can lead to a host of issues that you should be aware of. This article explains why running multiple OBD2 scan tools or devices at the same time on the same CAN bus is generally not recommended.
The core problem lies in how OBD2 scan devices communicate with your car’s computer, often referred to as the Powertrain Control Module (PCM). These devices don’t just passively listen; they actively request data. Each scan tool uses a specific “node ID” to initiate these data requests from the PCM.
Imagine multiple devices, each with its own node ID, all plugged into your OBD2 port via a splitter and all attempting to request data at the same time. The PCM, acting as the central communication hub, processes these requests in the order it receives them. However, the complication arises when the PCM sends the requested data back.
All connected OBD2 devices are listening for a response on the CAN bus. When the PCM transmits data, it’s intended for a specific device that made the request. But because multiple devices are connected and listening, they might all receive the data transmission. The crucial issue is that only one device is actually meant to receive and interpret that particular data packet.
This situation can lead to several undesirable outcomes. Firstly, some devices might display incorrect or nonsensical data because they are misinterpreting data intended for another device. Secondly, the confusion and data collisions on the CAN bus can cause one or more of your OBD2 devices to freeze, malfunction, or become unresponsive.
There’s a very specific and limited scenario where running two OBD2 devices might avoid these conflicts. This is when you have a vehicle with separate Engine Control Modules (ECM) and Transmission Control Modules (TCM), and you dedicate one OBD2 device to monitor only transmission data and the other to monitor only engine data. Vehicles like those with 6.0L, 6.4L, or 6.7L engines, which often have distinct ECM and TCM units, could potentially work in this manner.
The reason this might work in such cases is that the ECM and TCM typically use different CAN Node IDs. For instance, the ECM might use node ID 0x7E0, while the TCM uses 0x7E1. Because of these different IDs, there’s less chance of data intended for the ECM being misinterpreted by the TCM-monitoring device, and vice-versa.
Beyond data conflicts, there are also potential issues related to CAN bus termination when running multiple devices. Bus termination is a technical aspect of CAN bus networks that ensures signal integrity and prevents reflections. Adding multiple devices via a splitter could disrupt the intended bus termination, potentially causing further communication problems. However, that’s a more complex topic for another discussion.
In conclusion, while OBD2 splitters like a 3-way splitter allow you to physically connect multiple devices to your OBD2 port, it’s generally not advisable to run them simultaneously for standard scan tool operations. Expect unexpected results, data inaccuracies, and potential device malfunctions if you attempt to do so. It is better to use one OBD2 device at a time to ensure reliable and accurate communication with your vehicle’s computer system.
