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camerontech, Auto Service Technician
Category: Honda
Satisfied Customers: 433
Experience:  diagnostic auto tech
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99 Honda Civic lx, p0420 after aftermarket distributor

Customer Question

99 Honda Civic lx, p0420 after aftermarket distributor replacement. Car runs fine but p0420 won't clear. Need troubleshooting path. Cannot afford dealer
Submitted: 1 year ago.
Category: Honda
Expert:  camerontech replied 1 year ago.

hi there,

im cam a licenced diagnostic tech.

the p0420 is a catylist efficiency code.

I have seen this code many times on these engines and it has usually been the cat conveter.

a cracked exhaust manifold will also cause this code.

to check the cat...

montor the front and rear o2 sensors. if the rear o2 sensor is matching the front o2 sensor than the cat will need to be replaced

the front o2 should be toggling between .3 volts to .75 volts fairly quickly, while the rear o2 should be a steady slow wave volt range.

if the rear is toggling fast, that it finished.

this can be seen properly with a graphing scale on a diagnostic scanner.

you can use a multi meter volt scale but it will be a little slower to monitor as multimeters do not have a fast enough processor to catch every crosscount.

for me. it has always been a defective cat.

thanks and i appreciate positive ratings


Customer: replied 1 year ago.
I was thinking it was going to be the cat, I am going to look at those 02 sensors but I was already planning on maybe having to replace the cat.My next question/concern is if I do get a new cat and I haven't fixed the old problem i.e. defective igniter terminals on that new distributor that need crimping, will that burn up my new cat?Also was wondering if you have any experience with aftermarket cats in California and if the approved aftermarket cats are a reliable option?Thank You
Expert:  camerontech replied 1 year ago.

hi there.

sorry for the late reply... last of the xmas dinners.

regarding your new cat..

for the aftermarket cat, as long as its an "obd2" cat, and a good quality the cali emmission will be no problem. Its the bargain cats that will be a waste of money. spend extra as the obd2 cats will have larger elements inside and better chemical reaction qualities.

if you still have a misfire, this will lead to premature cat failure. If the engine light is flashing, this is a warning that immidiate cat damage is happening. the vehicle computer has pretty decent on board monitors that pick up problems.

if you have a decent scanner with misfire counter data screens and fuel trim data, you will be able to determine if you will have cat damaging issues or not.

if you need any more assistance id be happy to help!

please dont forget to rate me!



Expert:  camerontech replied 1 year ago.

hi there,

just a reminder to please rate me as this important for my page



Expert:  camerontech replied 1 year ago.


Customer: replied 1 year ago.
Cameron,So sorry for the delayed response as I've run into some trouble with my car. I decided to try and crimp the ingniter terminals on my distributor to rule out that being the cause and then proceed to troubleshooting my O2 sensors and then potentially a new cat. However after doing that I put the distributor back on and now the car won't start. It cranks but no start, ugh!!! I thought I might have installed the distributor in the wrong position so I brought the crank pulley to Tdc and then reinstalled my distributor. Again no start. This morning I was able to use my spark tester before I went to work and I discovered no spark at the plugs(at least on 1 and 2). I know none of this relates to my original question but do you have a direction I could take to continue troubleshooting? I will work back towards the battery and verify power at the coil, igniter etc. I was just wondering if you have any thoughts as to what I may have done during uninstalling/reinstalling my dist. My battery is just over 3 years old and was showing 12.3 v this morning when I looked at it. The car is one that hasn't been driven regularly for the past couple of months so the battery is definitely just sitting there and could be done. I just figured with 12.3 v that I would see some kind of spark so that leads me back to the distributor because that and the battery where the only things that were disconnected or reconnected. Sorry to run on but I wanted to give you as many details as possible. Thank you again for all of your help.

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