I have a 4 ltr x300 (1997), running on lpg. I had a bad misfire & thought it might be the coils because I changed the plugs only a few months back.
Because decent coils are so expensive I checked the coil connections with a small bulb across the connector (small wires) & they were all working.
Used a HEI tester on each coil - guess what - all working.
So I thought - sod it, let's change the plugs. Because I use it more on LPG, I changes the plugs for NGK BR6's, with a very small gap.
Result - no more misfire!!

 

A small update.

I have finally had a chance to give it a good run out at all speeds and for a decent time. No problems worth mentioning. There might be a tiny misfire but nothing to affect performance.

All I did was check a load of relays were soundly connected, take one coil off to look the plug and find that my plug spanner wouldn't fit down the hole - all the same I have six new plugs good to when I borrow a friend's slimmer spanner.

Because I bought it I have fitted a new fuel filter but I don't believe that has made any difference although the fuel lines are in terrible condition so that's going to start a new thread I suspect.

I cleared on 0413 code (it's UK spec so no relevant pump) and acquired a 1621 code which I will clear and see if it returns. The key fob didn't always work this afternoon but I'll put new batteries in.

So in summary I have a happy cat at the moment and I'm loving it.

I don't mind fixing things but I have never worked on a car with a computer on board before so it's a bit of a learning curve. In fact I relish getting in there and learning and fixing so long as I can fix it. At least we have a sensible chassis and the engine bay is accessible.

Mike