I have this pop up sporadically, when it is cold outside the past couple of years and it goes away and never presents itself in the summer. I am thinking if it was a cat issue, it would not be as temperamental. I am wondering if it is a winter gas mix they use up here in NY? Ever since I have owned it 44k miles I have used 87 Octane. I am around 137k right now and have seen it for the past ~10-20k miles. While I love this vehicle, I am getting annoyed at what I am having to do fix it. I am planning on selling it in a few months and getting a F-150. I wonder if I should have just ditched it when the tranny went out last year ><. I don't want to put much money into as there is no way I will get it back out.

A couple suggestions I have read:
1) Try higher octane fuel
2) Try seafoam
3) P0420 issue is more likely to be Cat failure. If it is a cat failure the Aftermarket cats are not as good as OEM.

I do have a couple questions:

1) Is there a way to test the 02 sensors?
2) Is there a way to confirm the cat is shot? It looks like it would be difficult to remove.

Thanks in advance!

 

cheapest would be to try using the recommended 91 octane and see if it goes away then.

 

Yeah "Ever since I have owned it 44k miles I have used 87 Octane" that is the reason why its failing, Did you missed the label in the gas cap that said PREMIUM Fuel Only ?

More likely its a 02 Sensor going bad sometimes they throw a code for Cat Failure but that is unlikely.. 02 Sensor might be fouled by the use of Regular, The only way to test them is with an Expensive OBDII Scanner that shows the Voltage on the Sensors.. Take it to an electric shop and they will diagnose in no time, If the Cat has indeed failed it will smell like rotten eggs on start up and the car will feel very sluggish (apart from the bogging of using Regular).

 

The crazy thing Thoiboi is that the 2nd Gen did not Recommend it, IT DEMANDED IT.. AFAIK all J37s were Premium Fuel Only.
It wasn't until the 3rd gen MDX that was "Recommended" when they went back to the J35.

 

i was giving him the benefit of the doubt


P0420 GENERALLY is cat failure... hopefully in your case it's only a matter of fouled up o2 sensor but you're not doing yourself any favors by running suboptimal octane gas..

 

Forgot to mention 2005 Acura MDX Touring

According to the manual and someone at Acura, the use of 87 is not required, but it is recommended to use a higher octane as it would get better MPG and HP.

A couple weeks ago I reset my it and I looked at the Fuel Trims with a friends OBDII scanner I borrowed. I did this for 1.5mins and recorded it for posterity.

The Long term trims were averaging around the following:
B1: .8 to 1.6
B2: -.8 to -1.6

The Short term trims were a little different.
B1: Peaked at 13 and hit 11 twice, though came back down stayed in the single digits, though a couple 1-2 higher than B2.
B2: Peaked at 5.5, though normally low digits

From what little I know it looks like the short term bank 1 is running more lean than bank 2. How hard is it to change the O2 sensor and what is the PN? I do change my brakes, I just don't know if I have time to take apart the whole car to get to one sensor.

 

Another thing to note, I have been having this issue for the past 3 winters, it seems to get slightly worst each year, though that could be me just being crazy. I just noticed not to long ago what sounds like harmonic resonance when I accelerate, like a heat shield or metallic rattle. I don't recall hearing it the past couple of years.

 

So maybe your cat is on its way out after all...

 

In the past 4 days the CEL went off and came on twice by itself. I filled my tank up yesterday morning, I will try it with premium, what octane do you guys suggest? It was off after I got out of work, when I was half way into the drive this morning the light came on, it is mid 30s right now.

 

dont think premium fuel would fix it.

you need to find the root cause of it and replace the issue.

 

Yeah, when I try to lookup posts on intermittent p0420 in cold weather, I see a lot of questions and suggestions, though people always forget to follow up to say "hey it worked", or "replaced the cat".

 

do you get any "rotten egg" smells?

when catalytic converters fail, they give off a sulfer smell.

 

also, I would change the o2 sensors first, before I would spend money on the cat.

Be sure to grab OEM sensors, as aftermarket 02 sensors will throw codes

 

Do you have emissions regulations in your state? If not get a test pipe and screw it..

 

No egg smells, it smells just like it always has. I think I will try some injector cleaner and some higher octane gas.

No, I live in NY, where everything is illegal.

If the injector and octane don't work, I will look into switching the O2 sensors. Any idea what O2 sensor I should change first? I read that some nicer OBDII scanners can read voltage of sensors. I have also read that some people just remove it use a torch to heat it up and read the voltage. I like to do as much as I can as shops just like to replace everything.

 

The concept of the heat torch is to sort 'clean' the O2 sensor because the thought is maybe it's funked up and not reading voltage properly. It's worth a shot. It would be bank 2 (post cat) that is reporting the cat below efficiency (P0420) code.

What I would do in your shoes is continue with top tier 91/93 octane gas (forget about the octane booster snake oil)
Pop off the O2 sensor and torch it up. Reset the code , drive and see if it fixes it.


One way to 'test' the catalytic converter is to knock it to see if you hear any loose media. That's indicative of cat failure.

(CA here so I feel your pain ) my last car had a p0420 and the only fix was the cat. I didn't bother replacing the O2 sensor, but did go OEM. They definitely last significantly longer than aftermarket . Get new exhaust gaskets with it and maybe bolts depending on if yours are rusted

 

Thanks, I am trying to avoid a lot of cost as I don't plan to have it much longer, time length in months. Spent $4k on a tranny and raditor, $800 in tires last year. Then just replaced the brakes and a caliper a month ago. This SUV hasn't been all that "reliable".

 

Honda/Acura vehicles are spot on when it comes to a 420 or 430 code. 9 out of 10 times you will need to replace your cat and it won't be an lambda sensor or secondary oxygen sensor.
The cats are of a non-continuous monitor which means that the program in charge of monitoring the efficiency of the cats does its tests only when the right parameters and criterias are present. Every make and model has it owns distinctive parameters.

I have seen vehicles where the cat monitor took weeks to set, specifically when the driver only commuted 5-10 minutes to work and back where the rest of the time the vehicle just sat. The program didn't have enough sample data to complete its final efficiency tests because the driving parameters were not met due to driving habits of the owner.

Cats love heat as this is when everything gets to work, normal operating temps can vary from 1200-1600 F. A degraded cat which is on the edge of failing may barely pass missing the DTC threshold in the summer, whereas in the winter it just cannot meet the standards due to colder temps.

 

Thanks for the info. I started parking it back in the garage and the light hasn't come on ever since I did that. The commute to work is greater than 30 minutes and this thanksgiving we did an 86 mile trip each way to the in laws and had no CEL. I also have been using premium gas 91-93, instead of 87. I am hoping this continues, though knowing my luck. I have spent more money on this vehicle than what it is worth in this past year, due to radiator issue that killed my tranny. Put new tires a couple months prior to that, just put on new premium brakes all around and a new caliper. Also replaced SAT DVD system for $75. I plan to sell it within this year and buy a F150, for an Acura/Honda, this vehicle has been less than reliable. The MDX is a nice vehicle and I like how it drives, I am just tired of having to fix a few things every year. The sad thing is I knew the transmission would need to be replaced, before I got rid of the vehicle. Though I was expecting the Torque converter to fail, not something with the radiator. I have taken good care of this vehicle and people are always surprised to find out how old the vehicle is. Maybe the next owner will have better luck with it.