I have a code come up which is B1265 on a 4.3 1996 GMC Jimmy

SOURCE: 95 gmc jimmy engine codes

9141 does not exist I will assume it is DTC P0141 HO2S Heater Performance Bank 1 Sensor 2

The VCM has the capability to monitor this process by using the Heated Oxygen Sensor (HO2S Bank 1, Sensor 2). The HO2S (Bank 1, Sensor 2), located in the exhaust stream before the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst; this in turn indicates the catalysts ability to convert the exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Heated Oxygen Sensor (Bank 1, Sensor 1).
When the HO2S temperature measures below 360°C (600°F), the sensor will not produce any voltage and will behave like an open circuit. This will result in an Open Loop operation.
The HO2S heater provides for a faster sensor warm-up which allows the sensor to become active in a shorter period of time and remain active during long extended idle. The DTC P0141 determines if the HO2S is functioning properly by checking the response time of the sensor. This DTC is a type B DTC.

SOURCE: Vehicle start shaking and died. Has a trouble code P0300

Error code P0300 means that you are having multiple/random cylinder misfires.  I would start with changing all of the spark plugs and see if that will fix it.  You will have to have the code reset at an auto parts store.  If that doesn't work let me know and we can progress down the line.

SOURCE: 1999 gmc jimmy 151k miles code 0452, emmissions help

THIS IS A FUEL TANK PRESSURE SENSOR CODE. MOST LIKELY THE SENSOR ITSELF IS BAD BUT CHECK THE WIRING AND THE ECM.

SOURCE: wath P1406 Code means on a GMC jimmy 1996,

It's an egr valve problem (exhaust gas return) egr position sensor.
Take off the egr valve and turn it over and see if there is carbon build up in the ports, if there is clean it out with a pipe cleaner and a little carbon cleaner, just spray a little into the ports and clean it out. If that doesn't cure the problem, then either the sensor needs replacing, and or the egr valve.

Testimonial: " thanks for yout help"