Which half of catalytic to replace first - front or rear?

[senor perfecto]

I’m getting a new cat and hoping to pass smog by replacing one not both. Mechanics are split on whether to try the front or rear first. What’s your thought on this any why?

FYI cataclean did nothing, and already replaced the O2 and F/A sensor. P0420 code plus CEL. This is for CA smog.

 

[phattyduck]

The front/main cat should do 'more' of the work. It is closer to the engine, so will heat up first and work first (good if you have a '99 that needs to go on the rollers).

-Charlie

 

[senor perfecto]

BTW my mechanic replaced the after cat O2 and the F/A sensor... he said there is only one O2 sensor but I'm seeing maybe there are 2?? How would he have missed that?

 

[phattyduck]

BTW my mechanic replaced the after cat O2 and the F/A sensor... he said there is only one O2 sensor but I'm seeing maybe there are 2?? How would he have missed that?

There are 2 O2 sensors on the Fed emissions trucks and 1 AF and one O2 on the CARB trucks. In either case, two total sensors in the exhaust.

-Charlie

 

[senor perfecto]

There are 2 O2 sensors on the Fed emissions trucks and 1 AF and one O2 on the CARB trucks. In either case, two total sensors in the exhaust.

-Charlie

Makes sense now! So since this is a California vehicle, he was right - one O2 (after cat) and one AF before. Both have been replaced and still getting P04210 and MIL, so I am still stuck - do I try replacing the front first? Is there any way to tell which one is bad via OBDII codes? Or just replace the front and hope for the best? I also have no idea if there is a better chance with a "better" cat like Magnaflow, or if the AP/Eastern ones (cheaper) are just as good at cleaning the emissions for a smog check?

 

[Jakeepoo3]

Makes sense now! So since this is a California vehicle, he was right - one O2 (after cat) and one AF before. Both have been replaced and still getting P04210 and MIL, so I am still stuck - do I try replacing the front first? Is there any way to tell which one is bad via OBDII codes? Or just replace the front and hope for the best? I also have no idea if there is a better chance with a "better" cat like Magnaflow, or if the AP/Eastern ones (cheaper) are just as good at cleaning the emissions for a smog check?

Hopefully the mechanic checked the O2 and a/f to PCM circuitry also. If that checks out both the cats are bad. If you're just wanting to put one back in personally I'd go with the one towards the rear. Keeps heat down near bay and give a little extra power. They don't do anything anyways if your girls a/f ratio (Stoichiometric mixture) is properly maintained, full combustion.. it's a law because human sheep don't understand sh!t they're riding on tune her up good before smog. Throttle body, related PCM inputs/sensors, injectors, plugs, etc.. you'll pass

 

[Dezert4Runner]

Replaced one of my cats recently (rear cat) after first determining I had no exhaust leaks. Borrowed a friends Harbor Freight infrared temp gun and found out the rear cat wasn’t operating right. There’s a few videos on Youtube showing a quick explanation of what you need to be looking for and where exactly to point the red dot to get the actual temp while avoiding the heat shroud. All at operating temp and with the vehicle running. If I remember right, the inlet temp should be noticeably cooler than the outlet temp if it’s operating normally. If it’s more or less the same or the inlet is hotter, the cat is bad. Hope that helps.

 

[Jakeepoo3]

Replaced one of my cats recently (rear cat) after first determining I had no exhaust leaks. Borrowed a friends Harbor Freight infrared temp gun and found out the rear cat wasn’t operating right. There’s a few videos on Youtube showing a quick explanation of what you need to be looking for and where exactly to point the red dot to get the actual temp while avoiding the heat shroud. All at operating temp and with the vehicle running. If I remember right, the inlet temp should be noticeably cooler than the outlet temp if it’s operating normally. If it’s more or less the same or the inlet is hotter, the cat is bad. Hope that helps.

That's a good testing method. Because the downstream O2 is monitoring both cats, which is dumb. Because it's purpose is to monitor the operating condition of the cats but won't tell you which one is bad. If you only had one cat you could easily hook up a live data scanner to monitor up and downstream sensor signals. Telling you for sure if it's good or bad. I would just assume they are both bad, precious scrap metal.. being a major pain to replace. Don't forget to keep the olds for some good scrap money.

 

[Devbot]

Hopefully the mechanic checked the O2 and a/f to PCM circuitry also. If that checks out both the cats are bad. If you're just wanting to put one back in personally I'd go with the one towards the rear. Keeps heat down near bay and give a little extra power. They don't do anything anyways if your girls a/f ratio (Stoichiometric mixture) is properly maintained, full combustion.. it's a law because human sheep don't understand sh!t they're riding on tune her up good before smog. Throttle body, related PCM inputs/sensors, injectors, plugs, etc.. you'll pass

Not true at all, they increase pollution! A few years back it was found that one of the byproducts of catalyzing NOx and CO2 is laughing gas, which is over 300x more damaging to the ozone than CO2 is. As it stands, by the EPA's own estimation, anywhere between 20-30% of the nitrous oxide in the atmosphere is produced by cars - directly from the equipment they themselves mandated.
Best of all when the EPA was questioned about these findings they shrugged their shoulders and said "hey we're in the business of improving air quality, not reducing pollution." Can't make this shit up.

 

[Jakeepoo3]

Not true at all, they increase pollution! A few years back it was found that one of the byproducts of catalyzing NOx and CO2 is laughing gas, which is over 300x more damaging to the ozone than CO2 is. As it stands, by the EPA's own estimation, anywhere between 20-30% of the nitrous oxide in the atmosphere is produced by cars - directly from the equipment they themselves mandated.
Best of all when the EPA was questioned about these findings they shrugged their shoulders and said "hey we're in the business of improving air quality, not reducing pollution." Can't make this shit up.

I agree it's a huge circle jerk. Yeah nitrous oxide is produced when the mixture temperature is excessive. It's supposed to be converted to pure nitrogen by the cat. Thankfully I'm in rural Arkansas where I cut the sl_ts out and straight piped her. O2 simulator wired directly to PCM. More horsepower, torque, acceleration, better mileage. Most likely less pollution because my a/f ratio stays perfect because of proper maintenance I control not a filthy cat. Not to mention the mining of precious metals destroying the planet. C(x)H+O2>H20+CO2, full combustion, basic organic chemistry. Notice the products are the same products we breathe out.. I reckon the government forgot to learn chemistry. I am not a human.