Change exhaust from dual cats to single?

[Supercharged111]

I've always been under the impression that the rear O2 sensors shouldn't effect the way the truck runs under any circumstances. It's common practice to disable them in the tune (or fool them in various ways like Kenny mentioned) if you don't want to run cats. I thought that the front O2 sensors handled all of the fuel adjustments.

@Supercharged111 This is the first time I've heard anyone mention that the rear O2s are capable of detecting an overtemp situation, how'd you figure that out?

I've definitely driven my '97s a few times with CELs for the rear O2 sensors, I don't recall any noticeable loss of power or anything that resembled a limp mode.

I forget exactly how, but years ago the way it was explained to me was that a narrowband uses a heat measurement to feed into its AFR calculation which is part of why it is only accurate at or near stoich. These trucks know how hot the cats are, and the O2 sensors are the only thing in the exhaust stream. My truck has 3 different temp thresholds where the truck will keep adding fuel to cool the cats. The old 2 way cats got rid of CO only, but a 3 way can also get rid of NOx, but it has to switch back and forth from rich to lean to get both. There is a wealth of information in the tune that helps you understand what makes the truck tick. This is one of those things.

Kenny I can't speak to your mpg loss, I've never seen anything that hints toward what you observed.

 

[Aarong23]

Bank 1 sensor 2 on my truck says something like, "heat circuit malfunction, no activity."
Changed it twice over that last 9 years and it's never fixed it. Lol
But it's never given many any trouble.

 

[kennythewelder]

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[PlayingWithTBI]

Good info, a little long winded but, worth it. I'd like to see the WBO2 video. Where can I find that?

 

[L31MaxExpress]

I had a check engine light from hollow cats on the 97 for a while. I did not hollow them out either. The ceramic came apart over time and blew out of the tailpipe. I had the catalyst oveheat protection turned off in the tune because it felt like I was dragging a box trailer on the highway when it enabled for miles at a time. My power was never down and the mileage was actually better when the cats destroyed themselves. The high flow metallic core cats and o2 extensions I put on seemed to have zero driveability change only turned the light off.

 

[Schurkey]

1. A vehicle that was emissions-approved with two cats cannot legally be switched to one cat. It may be that you don't care about legality, it may be that the folks doing emissions-testing won't notice...but unless you can find an EPA or CARB exemption, you need the same number of cats in the same location as the OEM parts.

2. Early cats reduced HC as well as CO. Three-way cats add NOx reduction.

Bank 1 sensor 2 on my truck says something like, "heat circuit malfunction, no activity."
Changed it twice over that last 9 years and it's never fixed it. Lol
But it's never given many any trouble.

Likely a wire-harness problem then. Possibly an ECM issue. One way or another, the O2 heater circuit seems faulty, which means the sensors will take longer to heat up as the only heat source will be the exhaust stream itself.

When I was getting heater-circuit faults, simply replacing the sensors took care of it. I guess I was lucky.

 

[L31MaxExpress]

1. A vehicle that was emissions-approved with two cats cannot legally be switched to one cat. It may be that you don't care about legality, it may be that the folks doing emissions-testing won't notice...but unless you can find an EPA or CARB exemption, you need the same number of cats in the same location as the OEM parts.

2. Early cats reduced HC as well as CO. Three-way cats add NOx reduction.


Likely a wire-harness problem then. Possibly an ECM issue. One way or another, the O2 heater circuit seems faulty, which means the sensors will take longer to heat up as the only heat source will be the exhaust stream itself.

When I was getting heater-circuit faults, simply replacing the sensors took care of it. I guess I was lucky.

I had to re-write the 305 engine tune I put into my 99 Tahoe in order to run the 350s dual cat exhaust system. The PCM was not having it until I made the necessary calibration changes. In all reality nobody knew it was a 305 except me so emissions test were easy to pass.