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The car has 120k miles. Since a month ago the engine light has been on, with code P0420. I brought it to autoshop, and was told that the code means catalyst converter is broken and it costs $800 to replace it. They said there is nothing abnormal except this and it is not worth replacing it, so they just reset it. They also said it is because of using 87 gas for too long. They suggested switching to premium gas. The trouble is that there may be issue at state inspection if the light turns on again later, and their suggestion is to reset the code one or two months before the inspection to build up history. Anyone has similar issues?
 

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Everything in Moderation
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P0420 means Catalyst Inefficiency. If COULD be the catalyst, but it could also be a contaminated oxygen (O2) sensor.

Have you had misfires or rough running in the past? Have the sparkplugs been replaced as Honda recommends at 100 - 110K miles? Were the valves adjusted at that time (also recommended by Honda)?


Since the O2 sensor is MUCH cheaper than the Cat, replace that first, and do the required 100K maintenance. MAKE SURE YOU USE A DENSO WIDE-BAND SENSOR. The cheap ones would allow the issue to come back soon after.

The advice about 87 VS 91 octane is bogus (unless they suspect you are using cheap gas).

You are correct about failing inspection if your CEL is illuminated.
 

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Discussion Starter #3
Thanks for the valuable information!

I haven't had any misfire or rough running. We usually drive with kids. The car has been running fine without any mechanical problems. We have engine oil changed at the dealer whenever reminder is on. We had brake pads and rotors replaced, transmission fluid flushed. We also had the spark plugs replaced at an auto shop (not the dealer) last year and drove about 10k miles after that. i did ask the shop about O2 sensor, but was told it is not related (they could be wrong). I have been using regular gas from Guilf, Shell, BP, etc. I still want to try premium gas for a little while. Hopefully it wont come back.

Anyone knows if this reset-one-month-before will work for inspection purpose?



P0420 means Catalyst Inefficiency. If COULD be the catalyst, but it could also be a contaminated oxygen (O2) sensor.

Have you had misfires or rough running in the past? Have the sparkplugs been replaced as Honda recommends at 100 - 110K miles? Were the valves adjusted at that time (also recommended by Honda)?


Since the O2 sensor is MUCH cheaper than the Cat, replace that first, and do the required 100K maintenance. MAKE SURE YOU USE A DENSO WIDE-BAND SENSOR. The cheap ones would allow the issue to come back soon after.

The advice about 87 VS 91 octane is bogus (unless they suspect you are using cheap gas).

You are correct about failing inspection if your CEL is illuminated.
 

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Everything in Moderation
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The reason that premium gas was recommended is: it is said to contain extra detergents. The name-brand stations you are using are Top Tier however, so you are doing fine there!

I recommend having the valves adjusted (unless you want to do them yourself).

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When the ECU is reset to turn off codes, it needs to 're-learn' many parameters over several drive cycles (not miles). If these parameters are not fully processed, it will trigger a 'Not Ready' indication when the inspector hooks up their scan tool, and you will fail.
 

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The check engine light comes on every two to three weeks. I put in a system cleaner and it goes away. Other times if I do not have time to immediately put a system cleaner in it just goes away. Both P0420 and P0139 codes show up. What is going on?
 

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Replace the rear 02 sensor. Done. If yohr getting 0420 it could be the cat and not the 02x but with the 0139 as well, rear 02 sensor is bad. "Bank 1" (4 cyl engines only have 1 bank, will see bank 2 on v6/v8 etc) but sensor 2 is the 02 sensor in or after the converter. Very straight forward on this one

Do NOT buy some cheap thing. Best to get a Denso sensor, Hondas are very pick about which spec 02 sensors you install.

Sent from my SM-G950U using Tapatalk
 
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