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Zerohour 10-02-2010 11:26 AM

Spark Plug Wire Resistance
I'm trying to troubleshoot my brother's Subaru Wagon's absurd issue with a P0420 Code.

The Subaru has a Catalytic converter loss of efficiency code. Although it has a new catalytic converter on the car. had been replaced previously by a shop. I have a feeling the previous owner got hosed in this situation, and since my brother now has the car and its getting the same code again, I'm trying to track down all the other issues.

Somewhere along the line, the shop changed the plug wires over to an "STI" set of wires. Except this STI is for Spark Transfer International, a small company that produces "HP adding wires". Needless to say the wires measure in at 1/4 of there factory spec. I'm concerned that these things are causing either poor spark, bad spark, or RF interference.

Does anyone here on have experience with this? I've changed enough spark plug wires in my time, but never bothered to chance "performance" plug wires.

Factory spec are 5.6 Kohm to 12.x Kohm
His wires are 1.4 Kohm to 2.0 Kohm

Any ideas, suggestions? The car's engine runs well, but I'm not sure if the wires are aiding a solution to the problem and I would hate to have him buy new plug wires for no reason. The poor kid already put the money out for a pair of O2 sensors. :mad:

brucey 10-02-2010 01:20 PM

I've got the PO420 code myself. I just keep clearing it with the scangauge, although you can use some anti-foulers for spark plugs and move the sensor out of the air stream, and a lot of people say that will fix it as well. It's just not worth fixing properly with the price of a genuine catalytic converter (the only thing guaranteed to work)

As for wire resistance, I'm not sure. I do know I broke a wire on my samurai one time and I just soldered it back together, and it actually ran better with that rigging job.

Burned 10-21-2010 12:19 AM

Its not always what it seems
Sometimes a faulty O2 sensor is not bad enough to set an O2 sensor code but is off just enough to affect the accuracy of the catalyst monitor.

pounsfos 10-21-2010 04:54 AM

Code P0420 is a code for Catalyst efficiency. The computer is seeing the post Catalytic converter o2 sensor having too much activity. The Cat. should be cleaning the exaust of Hc, co, and Nox. The post cat o2 will read the o2 content in the exhaust and if it moves up and down too much or to high and low, the computer interperates that as the cat not ridding the exhaust of it's pollutants. This indicates that the converter is unable to preform properly. The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail. Many other engine operating system failures can cause the converter to not work properly.

(your could be running to rich, if i was you i'd move to nz where you dont need a cat :), i've never had a cat converter in any of my cars haha theyre just extra weight to me)

oh and get some better spark plug wires, i use eagle leads but bosch ones have very low resistnace, 8mm at least

bestclimb 10-22-2010 12:45 AM

If plug wires are contributing, (which I doubt it as the majority of resistance is in the gap of the plug) you could get a set from a junk yard for peanuts, then sell them or the performance ones on Ebay.

winkosmosis 10-25-2010 04:57 AM

The STI wires are lower resistance, which is a good thing. I don't know what the quality is, but the ohm ratings alone don't indicate that they suck.

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