U0278 means a master module on the vehicle network (typically the PCM or a body/gateway module) has stopped receiving messages from the Seat Control Module designated 'L' (left or driver-side, depending on manufacturer). The code indicates a communication fault on the data bus, not an engine performance problem, and the seat module is being polled but is not responding.
Harnesses under seats flex and chafe from constant seat travel, and the connectors can work loose, corrode, or get contaminated with debris. This is by far the most common cause of U0278.
If the module loses its ignition feed, battery feed, or ground reference, it cannot power up and respond on the network even if its internal circuitry is fine. The relevant fuse and ground point should be checked.
Internal electronic failure of the seat module is possible, especially after water intrusion, physical impact, or aftermarket seat modifications, but it is less common than wiring or supply faults.
An open, short to power or ground, or high-resistance break in the CAN high/low lines between the gateway and the seat module, or a termination problem on that segment, can silence the module on the bus.
A shorted module on the same CAN segment can prevent the seat module from being seen. This is usually accompanied by multiple U-codes and modules that fail to respond to a scan tool.
The engine computer is only reporting what it sees on the network. Internal failure of the PCM or gateway is sometimes suggested for U-codes but is almost never the actual cause of U0278.
Yes, in nearly all cases. This is a body/comfort module communication issue and does not affect engine, transmission, or braking operation. Seat functions on the affected side may be inoperative.
It depends on the jurisdiction. Some regions fail any vehicle with a stored DTC, while others ignore non-emissions U-codes. Check the local inspection rules.
No. The PCM is only reporting that it cannot reach the seat control module; it is almost never the actual cause. Replacing or reflashing the PCM for this code is a common but usually incorrect recommendation. The real fault is nearly always in the seat wiring, connectors, power supply, or the seat module itself.
No. The seat control module is usually a small electronic unit mounted to the seat frame or integrated into a switch cluster, and the harness and connector are individually serviceable. Seat replacement is not normally required.