from sitting still, I start driving, RPM go very high and hang there before suddenly shifting so hard it feels like hitting a speed bump. Trans light starts flashing.

I take it to the shop while the light is still flashing and the computer says: "No Codes" They say they have no idea what is up and we drive it and every shift is really hard.

One guy says did you notice how hot this car is? There is a ton of heat coming from under the car and the engine bay (temp gauge is fine). He says take it to get catalytic converters checked.

I run it to a shop and the guy takes 3 seconds and shoots some laser thermometer at my exhaust just before and after the CC and says yup it is bad because it is hotter in front of the CC instead of behind it.

So my question.. Is there any way a Catalytic Converter issue can cause the transmission issue? When I restart the car, the tranny resets and drives normal. after a few weeks, this whole thing will happen again where the RPMs go up, it hard shifts, the light is back on, and the shifting is hard again until I turn the car off and back on....

2001 Isuzu rodeo 4 cyl 140k miles

 

That sounds more like if it was the Isuzu transmission range sensor. When it is on the verge of failure. Most cases the engine light will come on but sometimes it is intermittent. If at the time of hard shifts check to see if your shift selector indicator light goes off and on or stays off.

Otherwise this is a weird one if you have installed an aftermarket alarm..
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Information IB04-03-S001
Misfire DTCs and Catalytic Converter Damage Due to Installation of Alarm Systems
Issue Date: February 2004
Affected Vehicles
2003 - 2004 and prior Isuzu models.
Service Information
It has been determined that aftermarket alarm systems incorrectly installed in vehicles have the potential to cause misfire codes and damage the catalytic converter.
These alarm systems use a circuit interrupt which utilize the ignition circuit on the vehicle, they also utilize mechanical relays and normal vehicle movement can trigger these relays to engage and disengage the ignition circuit while the vehicle is in motion. These disruptions of the ignition circuit, which occur in milliseconds, may cause more fuel to be commanded. Over time, on and off fuel dumping can cause misfire codes and ultimately damage the converter assembly.
Engineering could not identify any alarms that utilize solid-state circuitry that would eliminate this concern. Because of this, it has been determined that all alarm systems must be routed through the starter circuit in order to avoid this condition.
Dealers must be aware of this issue and take note of the wiring on vehicles with alarm systems that come in for repair, particularly for catalytic converter damage that seem to have no known root cause.
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Of course it may not be the case. No TSB's found on the CC. If it does test bad it may have just failed all by itself.
You could have more than one issue if the vehicle shifts hard and the cat is bad.

In some cases if the cat is clogged it will cause some vehicles to lag or feel as though it is shifting rough as the air from the engine is not passing and shooting back to the engine.

 

by "shift selector indicator light" you just mean the highlighted 1,2,3,D,R light?

That never seems to change. The RPMs do seem to be getting higher and higher before it shifts. I asked the transmission shop about the range sensor, and they said when they replaces the transmission last year, they checked it and it checked out fine.

It is so weird that the "check trans" light would be flashing and at the same time relaying "No Codes" to the scanner.