Remote fob not working?

So many problems - sometimes very simple cures.

Vectra B/Omega B with separate fob. Fit a new battery and re-synchronise the fob by turning the ignition on and pressing the lock button. If the fob is working, it will lock and unlock the vehicle.

Astra G/Zafira/later Vectra B/Omega B (fob built into the key). Slide the fob out of the key using a small screwdriver in the slot. Open the fob with the small screwdriver and be prepared for the battery and its holder to fall out of the tiny circuit board. This would normally be time for a new fob - but a very basic fix is to cut a small piece of card or paper and fold it up to sit tight against the battery. This usually maintains contact within the fob and lets you get a couple more years out of it. Again, re-synchronise the fob if need be.

Corsa C. Usually very reliable and probably only needs a battery. Slide it out of the fob, open the fob gently (lugs either side) and replace the battery. Re-synchronise as above.

Astra H. Replacement of the battery is the usual cure but check for any signs of corrosion in the fob which will usually ruin it.

Vectra C. Early units are similar to the Vectra B and the later ones took on the Astra H style. Replace battery and re-synchronise.

If, after replacing batteries, the fobs still don't work, it is worth spraying WD40 over the printed circuit board and wiping it clean. That stuff is absolute magic and has got many a fob working for me again.