Hey,

I have a '99 Honda Accord (4 cylinder VTEC) and I had a new gas tank fill-up last week. Ever since then, my car has started to hesistate when I try to accelerate, although it does this only occasionally. Moreover, sometimes my car's idle is rough, like the rpm nominally stays around 800 and it suddenly falls to 300 or so and recovers to 800 in 1 or 2 seconds.

A few days after all this, the check engine light came on with P0420 code (and its the only code). Although the code says that the catalytic converter efficiency is below threshold and something maybe wrong with it, I am not sure if a bad catalytic converter can cause my car to sometimes hesitate and exhibit rough idling.

Could P0420 appear if the gasoline I filled-up with was of poor quality?

I would really appreciate every advice from you.
Thank you!

 

1st. hondas ... hondas run best with 87 pump gas ... well most hondas ... accords being one of them

2nd. if the cat is broken it will run sluggish ... i would check the cat, make sure its intact inside ... if its still good, make sure that you have no exhaust leaks ... if all fails i'd try changing the primary o2 sensor ... i know it sucks, i had to go through the same deal

 

If she has high mileage more then likely its just the cat. However that code may also come up if the rear O2 isnt doing its job and is stuck more so on rich making the computer think the cat isnt working. 9 times out of 10 its just the cat, however i usually tell my customers it would be best to just replace any old O2 sensors with a cat job. Old O2 sensors are like old sparkplugs yeah they work but are they really as efficient as they could be? Just be sure to replace with honda factory replacement parts, dont cheap out on this stuff. Hope this helps you understand a little better.

 

The P0420 is a code set when the secondary O2 sensors' values agree with the Primary O2 sensor. They aren't supposed to. Curiously, the paradox comes from determining if the converter has gone 'south' or if the secondary O2 sensor is lying.
In most cases, the O2 Sensor is Lying.
They can fail from age.
They can fail from contaminates.
They can fail from damage to their wiring.

The fuel question is a valid one; but in order to do damage it would have had to have the quality of Biodiesel mixed with swamp water. (get the idea?)

Unfortunately, neither the Catalitic Converter or the Secondary O2 sensor will cause an idle / hesitation complaint.
The Primary O2 sensor could be (and sometimes is) the culprit, but you have no code to indicate a failure (yet).

P

 

The last time my car received a P0420, it was in tandem with a P0505 (IACV error) It might just be a fluke, because after I cleared mine, the P0420 went away and never came back (but the P0505 did come back though =() Do you have any electrical problems? Like for example symptoms of a failing alternator? Sometimes (like in my case) a failing alternator can throw false positive OBD2 codes. (turns out my IACV and my cat converter is fine)

 

I appreciate everyone's input. So apparently the hesitation and rough idling went away after changing the spark plugs. But the check engine light was still on. I then took it to a local mechanic and he took the voltage reading from both O2 sensors on his machine and saw very little signal level on both of them.

He said the O2 sensor before the CAT is definitely bad since it's signal output was so low. He also siggested changing the O2 sensor after the CAT, but b'cse he didn't say that it's bad, I haven't changed it yet. I changed the O2 sensor before the CAT (Bosch and one for LEV Accord), and reset the check engine light. I drove around for a while and it didn't come back on. Before it used to come back on after driving for only a few miles after resetting.

However, it has now come back ON after driving for 540 miles! Do you think I should also change the O2 sensor after the CAT? Or is my CAT that's bad?

Again, I really appreciate all the help.