I worked in a Vauxhall dealership from 1980 - 1995, and then was a roadside patrol until 2007. There's one thing in particular I learned in that time from a huge amount of experience, and that is Vauxhalls do not like non-genuine spark plugs. They're fine for the first few hundred miles, but after that they can be difficult to start from cold and in the damp. If you have NGK or Champion plugs chuck them away and fit genuine Bosch plugs from a dealer. There is a scientific reason for this. It's to do with the internal resistance of the plug. Some might not believe me, but I have attended hundreds of Vauxhall cars struggling to start in the morning which have had non-genuine plugs fitted. Replacing them with genuine cures it every time.

Having said that this may not apply to newer models.

Not being familiar anymore with specific models, how many coils does your engine have? Do you have one coil pack for all four cylinders, two coils each for a pair of cylinders, or one coil for each cylinder individually?

With the exception of coil per cylinder, bear in mind that cylinders 1 and 4 will fire together, and so will 2 and 3. If you have a misfire on 2 and 3 it could be that either the 12v supply for the coils that fire those two cylinders could be missing due to a broken wire, or the coil negative to the ECM wire could be broken. Check for continuity between the coil and ECM. Or the transistor that switches coils 2 and 3 inside the ECM could be blown which means you'll need to either get it repaired by a specialist or replaced.

It will then be four coils in one pack. Each coil fires one cylinder. On the multi plug if there are six wires, one will be coil positive common to all four, one will be earth, and four will be each of the coil negatives to the ECM.

Remove the coil pack from the engine but leave the multiplug connected. Fit a spark plug to each of the four coils and lay it on top of the engine. If you can use a suitable piece of wire to earth the body of each plug. Then get an assistant to crank the engine while you look to see if each plug is sparking. Before you do though I would pull the fuel pump fuse or relay to prevent the fuel pump from running.

If only two spark plugs are sparking then you'll need to check the wiring to see if you have a live feed to all four coils. If you have then you'll need to check to see if all four are switching. The feed to all four coils probably comes from one common wire, or they may be fed in pairs. That's why you'll need to check. You'll either have one live with ignition on at the plug, or there may be two. You'll have to figure that out.

To check for coil switching use a voltmeter. Connect between each switching wire in turn and earth. Crank the engine and read the voltage. It will switch between 12v and 0v. BUT, a digital voltmeter cannot react quickly enough to this constant change back and forth, so it will sit somewhere in between, eg around 2v or 4v. There is not an exact figure, but if it stays on 12v or 0v then the coil is not switching. In this case check the wiring for continuity from the coil multiplug to the ECM multiplug. If there's a break in the wiring you'll need to trace and repair it, and if the wiring's good then a transistor has gone down inside the ECM.