If the CEL comes on at key on and then goes out again then the ECU doesn't know that the cat is not there and the fuelling won't be affected.

Just got back from Oregon,going through the syskhous and flatland averaged 26.1 mpg,very happy

If the CEL comes on at key on and then goes out again then the ECU doesn't know that the cat is not there and the fuelling won't be affected.

OBDI ECU's have no way to determine the cats condition, this was addressed with OBDII when they added the secondary o2 sensor. The secondary heated o2 sensors purpose is to read the temp of the exhaust gases AFTER the cat, the ECU then compares that reading to the primary sensor reading to verify a temp drop indicating the cat is operating and catalizing unburned fuel and byproducts.

A simple work around for those driving strictly offroad and wish to remove the cat while not getting a CEL, is to remove the secondary sensor from direct contact with the exhaust stream. This is accomplished by creating a standoff, go to any autoparts store like Auto-Zone and look for a set of sparkplug defoulers(blister pack with 2 defoulers), take one of the defoulers and drill out the restricted end so that the o2 sensor will fit without touching the sides on the defouler, thread the second unmolested defouler on and then install in the secondaries bung.

The problem is nothing to do with the condition of the cat but whether enough back pressure remains for the EGR to still function, the ECU looks for a change in intake manifold pressure when the EGR opens. The ECU will default to back up settings if it thinks that the EGR is faulty, that is what the earlier poster was alluding to. If the CEL is not on then the ECU won't be altering the fuelling.
On an OBD2 ECU the second O2 sensor checks for a change in the oxygen level not the temperature, the sensors are heated with a PTC element that regulates its own temperature.

But the sensors determine the o2 levels by changing resistance within the sensor material due to temperature(reaction to o2 in exhaust), I have used this mod succesfully many, many times on OBDII vehicles. By standing it off but not removing it entirely from the exhaust flow it tricks the secondary sensor into outputting a voltage that is within the range expected by the ECU with a working cat. In fact, there are even companies that are making them all fancy dancy specially machined, but charge a heck of a lot more than the defoulers and they both do the same job.

But the sensors determine the o2 levels by changing resistance within the sensor material due to temperature(reaction to o2 in exhaust).

No they don't, they use a Nernst cell which is an electrochemical cell which generates a voltage due to the absorption of oxygen atoms, the resistance doesn't change. Spacing the second sensor is supposed to move the sensor out of the exhaust stream and shield it from the exhaust gasses.
None of which is relevant because no one was talking about the ECU detecting the removal of the cat  only the possible non-functioning of the EGR.

Quote from: talonxracer on January 04, 2012, 03:40:47 PM

But the sensors determine the o2 levels by changing resistance within the sensor material due to temperature(reaction to o2 in exhaust).

No they don't, they use a Nernst cell which is an electrochemical cell which generates a voltage due to the absorption of oxygen atoms, the resistance doesn't change. Spacing the second sensor is supposed to move the sensor out of the exhaust stream and shield it from the exhaust gasses.
None of which is relevant because no one was talking about the ECU detecting the removal of the cat  only the possible non-functioning of the EGR.

You are correct, I dont know why I had that in my noggin,,,messing with those darn snomobiles too much probably, LOL...

And if the rest of the exhaust is stock the removal of the cat shouldnt cause an issue with the EGR