For months I have been troubled with this problem that has been very annoying and hopefully help anyone with the same problem.

The engine light had been coming on, usually when off throttle and the car is slowing or engine braking but when dip the clutch to let the car idle the fault clears.

Fault code was 24 idle control system. If you try investigating, the most common fix would seem to be MAF or IACV. I tried changing both but made no difference.

After some further research via the workshop manuals listed on here, I realised that the gearbox has a "neutral switch" connected to the fuel injection system. so I thought it was worth testing this switch as the car was perfectly happy once back to idle.

Anyway remove sensor, rearmost of the two on passenger side from under car and has two pins and is a simple push button switch.

Test continuity between the two.
Gearbox neutral should give open circuit
Gearbox in gear should give closed circuit

Mine was constantly open circuit and so was obviously faulty, has been replaced and is now running much better without engine light appearing once. It actually made more of a difference than I expected, when slowing to a stop and pressing clutch, the revs used to drop very low, I thought this was normal but apparently not and has eliminated that too.

1999 type R but im pretty sure will relate to all classics and possibly newage but diagnostics may be better at locating this fault

 

Good info.