2003 Trailblazer reduced engine power after replacing spark plugs

just replaced spark plugs and now it is running rough and reduced engine power light is on. did not mix up wires or coil packs. did one at a time.
any suggestions?

Have you double checked your spark plug part numbers?
Are you using OEM replacements or “designer” plugs?
Have you gone back over your work?

yes on all counts! triple checked! (used OEM)


don’t know the codes, don’t have computer and can’t drive it to where computer is. It won’t let you put it in gear

And the spark plug gap. Did you check that? Many plugs do need to be gapped to spec before installing. Did you happen to use an anti-seize and perhaps glop too much on there?

Are they the original wires? I don’t know how many miles are on them, but they do have about 12 years on them. Not only do they degrade over time, in general, but the terminals fatigue and can lose their “spring.” So after having been pulled they may not reseat as well as they should. The general abuse of pulling and reseating, even if you do it right and work the boots only (did you pull on the wires themselves or just the boots?), can compromise the connections.

this is a straight line 6, there are no real plug wires to speak of. the spark plugs are gapped correctly. the plugs sit down inside the block. the way the wiring harness is setup cant really mess up the order of the plugins

Did the plugs screw in easily and readily or did you have to use some elbow grease?
How did you torque them?

I cannot help but suspect that the plugs aren’t seated properly.

we are getting ready to check them again. i read something about taking off the boot from the coil pack and putting it on the plug then putting the coil pack on


Did you mess around with the electronic throttle body for any reason?

Did you unplug the connector or move it out of the way to do the plugs?

The boots on there are replaceable (at least I’ve seen the kits). I would also inspect carefully for cracks in any of the coils and inspect at any stress points on the wiring harness for signs of damage. If your check engine light has come on (it should eventually if not already) reading the codes can probably pinpoint the cylinders with a problem (P030x codes), or tell you that it’s more likely upstream in the wiring harness (P0300).

did not mess with throttle body
we did disconnect the coil packs, we have double checked to make sure they were plugged back in properly

Won’t let you put it in gear? Don’t understand what that means.

I’m guessing you may have “missed” a spark plug with the boot, meaning you may have the boot next to the spark plug instead of on it. And as noted above, you do have the throttle assy plugged in properly, right?

Replace the coil pack boots.


As with old spark plug wires, if they’re messed with they no longer provide a good path of high voltage to the spark plugs.