foumer

What up peeps? Does anyone know if the newer hondas with maf sensors use the downstream O2 sensor to trim fuel ? My accord was giving me a P0420 code for the catalytic so I used a spark plug defouler on the downstream O2 to get rid of the code.

Doodasi

I guess you could look at it this way. Short term fuel trim is front o2, and rear is long term. If you delete rear on cat monitoring system, your block learn will be all screwed up. Correct me if I'm wrong.

JRCivic1

If it is a 4 cylinder application, the front sensor is for fuel trim (both short term and long term) and the rear (downstream) sensor if for catalyst efficiency.

Doodasi

I took a catalytic converter for dummies class a long time ago, and the instructor tried to teach me something like what I said. I think I'm right about that. Instructors do come up with goofy stuff sometimes though. Like one time a guy told me that washing a new brake rotor in soapy water demagnatizes it

Apparently your cell number (1-32) is chosen by your rear o2 in a cat monitoring system. That's why you can't just take the cat out, or remove the sensor. Anything over or under +-16 will make a lean/rich long term fuel trim code. If your front (STFT) is +-16 it changes your LTFT block learn cell number.
p0420 can't make the two work together. If you put a resistor in place of your rear o2, it will never run right.

Correct me if I'm wrong. Learning about fuel trim, and cat efficiency is not quick dirty and easy.

JRCivic1

Doodasi, you are not completely wrong... just wrong about a Honda doing it. Beginning in the late 80's to early 90's, many German car manufacturers actually used the post cat O2 sensor to alter the A/F slightly to improve catalyst efficiency. This did not affect the primary O2 sensor's closed loop operation, and it was not considered a change to either the short term or long term fuel trims... but merely an additional alternate trim for optimum catalyst performance.

In a '96-01 Honda OBD-2 type emissions system, the rear O2 sensor merely analyzes the oxygen content of the exhaust after the cat, and compares this value with that of the primary (upstream) sensor. If the values match, the ECU believes that the cat is malfunctioning, and this is why a simple change in the exhaust flow/turbulence by using a fouler or signal restrictor may defeat the P0420 code.

Newer applications that have a wide band primary O2 sensor and a narrow band post cat/secondary O2 sensor use a different method to optimize catalyst efficiency. The ECU will expect specific readings based on the engine coolant temperature... and if these values are not within a specific range, the ECU believes the cat to be malfunctioning and throws the P0420 code.

So you are spot on when you say that fuel trim and cat efficiency are not quick, dirty and easy.

foumer

Good stuff. I am buying a new downstream O2 sensor soon. I will try it without the defouler. Currently I am getting about 29 mpg 30% city/ 70% highway with the defouler, reported through the on board mpg display.

Doodasi

the converter is most likely your issue. Take it out, and inspect it. If it looks o.k., and no substrate material is falling out of it, soak it in a tub of soapy dishwater for a day. Sometimes that works.

tech8

Depending on where you live and the mileage on the car, the converter may be covered under the Federal emission control warranty of 8 years/ 80,000 miles.

foumer

Bought the car with 162k miles so it could be either, but most likely its the cat like you said. I looked at youtube videos about washing it dish detergent, but since its too cold already to mess with that, I am going to do after winter. But I will try a new o2 for reference.
Car has 2 catalytic converters. I might just get rid of the second one since its after the downstream O2.

foumer

I am out of warranty, it has 162k miles. Previous owner practically lived in the car but I got it really cheap so I cant complain.

DCFIVER

No,he was correct. Honda has been utilizing the rear O2 for trim corrections(for catalyst efficiency) since at least the mid 90s.(well,96 on) It was the internets' misconception(and lack of widely published material) that spread the lie that the rear O2 was only used for catalyst monitoring.

DCFIVER

Yes they do,but the authority of the rear O2 is somewhere in the neighborhood of 5%.

Meaning its trim corrections will not adversely affect MPG one way or the other.

DCFIVER

It really depends on the amount of authority given to the rear O2. Different manufactures allowed different amounts. It also varies by year and model.

In Hondas case they have never given more than a few percentage points to the rear O2. Its trim purpose is ensuring proper feed-gas for the Cat, not fuel economy or engine performance.

foumer

Thanks for the input. Just for testing purposes I will try a new downstream O2 (without the defouler) with the cat they way it is and check the mpg. If it still the same I will wait until it gets warmer out and try to clean the converter and try again.

DCFIVER

Cleaning it wont do anything. If the vehicle has over 100k,the cat is done. They are relatively small and subjected to intense heat cycling. Eventually they just wear out. No amount of "cleaning" will work.

The defoulers are hit and miss. Sometimes they work,sometimes they dont,not sure as to the reasons for the variations,though I suspect software plays a major role in it.

For what its worth,my '04 RSX has had a defouler for 6 years now....

foumer

I had a defouler on my rsx too for a couple of years and no light.. lol.. I have been driving the accord for almost 3 weeks with the defouler and no light yet.