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The transmission module unable to reach the engine module across the network. The transmission depends on the engine module for almost everything it uses to decide a shift — throttle position, engine speed, torque, temperature — so losing that conversation leaves it with no basis for a decision.

The response is a fixed default, which is why the vehicle is still driveable. The transmission holds one gear, usually third, so the vehicle can be moved without shifting on information it does not have. A car stuck in one gear with a heavy take-off and no upshifts is describing this, and it is a protective state rather than a mechanical failure.

The fault is rarely in either module. A bus wiring fault, a poor earth, a low supply voltage or another module disrupting the network will all produce it — and the strongest clue is other lost-communication codes stored elsewhere naming other modules. Read every module's codes rather than the transmission's alone: one module reporting a loss is a link, several reporting it is the network, and that distinction decides where to start.