When do the spark plugs need changing?

Hi Bronze. I just changed mine for the first time last week. This was at 200,000 km (125000 miles) and I was amazed how good the plugs I removed looked.

Anyway it's a pretty easy DIY job, so if you want to replace them a bit early then go for it.

 

The recommended change interval is at 120K miles. However if you notice engine misfire then it would not hurt to remove and inspect (or replace) the spark plugs now. Use only the correct NGK or Denso iridium spark plugs.

You may find moisture in a spark plug well which would cause misfire. The spark igniter O-ring does not do a very good job of sealing the igniter opening.

Water can drip down from the engine compartment cowl, onto the engine valve cover. If this is happening, use black sealant on the vertical seam between the two plastic cowl cover pieces.

 

Martin, Patrick has covered it all really, but just a couple of extra checks.
Check the throttle body is clean.
I have also noticed if I move the car when the engine is cold and do not allow it to warm up "not being quick enough with the EV button or forgetting it" turning it off after moving the car a few yards, next morning I can have a a short period of rough running "missfire". The gen3 Prius seems to have real problems with this, but it is a thing I notice in the gen2 to a smaller degree when it is cold. After 6 years with my car the problem "if you can call it that" has got no worse.

John (Britprius)

 

Could it be because of the variable valve timming? H