Code p1294

P1294 - Target Idle Not Reached

You've gotten some misinformation. No engine has had an idle adjusting screw since the mid to late '80s. In fact, idle speed is adjusted by the "idle air control" motor; (IAC), what you referred to as a sensor. It's also commonly called the "automatic idle speed" motor, (AIS). This is not a spinning motor as you'd think of. It is a "stepper" motor. It's armature is slowly placed to one of 256 positions by varying voltages and polarities supplied by the Engine Computer. Those positions are called "steps". They can be read on a scanner. For a properly-running engine, step 32 is typical. For one misfiring cylinder on a V-8 engine, you'll find it near step 50. As the armature slowly rotates, it is on a threaded shaft that retracts a pintle valve. That valve opens an air passage around the throttle blade to let more air in. At the same time, it increases the length of time it pulses the injectors open to also increase fuel. More air and fuel increases idle speed.

By far the most common problem is after the battery or Engine Computer has been disconnected, the computer's memory is lost. Most of that data gets rebuilt as soon as you start driving, without you even noticing, except for "minimum throttle". That requires a very specific set of conditions to be met for the relearn to take place. Until that is done, the engine may not start and run unless the accelerator pedal is held down 1/4", you won't get the normal "idle flare-up" to 1500 rpm at start-up, and it will tend to stall at stop signs. When minimum throttle hasn't been relearned yet, the scanner will show it at step 0.

To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

The clue was when you said idle speed drops to 300 - 400 rpm. The computer is more than capable of making that up once minimum throttle has been relearned. There is no other means of adjusting idle speed.