Bought a 98 s10 4x4 thought it ran ok but now that I've driven it I need some help. After warm up it will stall intermittently at stop lights and will jerk a few times between 3 to 8 miles into driving but not after that. Codes 0118 and 0420 have led me to discover that the Cat. Converter has been removed, replaced with straight pipe and I can't find the temp sensor by the t stat or its wiring? I'm guessing the temp sensor would help cpu keep fuel flow correct and stop the stalling. Shouldn't the wiring for the sensor be with the egr wiring or nearby? The Temp gauge works in the cluster, at times when motor starts sputtering it bounces around? Also have fuel gauge that doesn't read correctly but will check on that later.

 

P0118 - Engine Coolant Temperature [ECT] Sensor Circuit High Voltage
The temp sensor on the 98+ is in the driver side cylinder head.
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
P0420 would definitely come up if you didn't have a catalytic converter installed.

What general maintenance have you done? Things like cleaning the MAF sensor, throttle body (including IAC bore/valve) and the EGR valve are quite easy and of minimal expense, but can have a profound effect on how your engine runs.

Here are some threads from the Tech Article (DIY) section that can help with those tasks:
MAF Sensor Cleaning
How to remove Throttle Body 2nd Gen 4.3
EGR Valve Cleaning

 

Thanks Swartlkk for sensor location and maintenance tips. I have cleaned the egr valve (wasn't as built up with carbon as the pics in the procedure), also inspected the MAF, looked clean but will check again. I have not removed the throttle body yet but will do. Funny thing about the P0118 code, took truck to a guy with a obd-ll scanner pulled up the same codes P0118 and P0420 but he was able to read the temperature as it increased and got to stat rating? Seems if P0118 code was real he would not have been able to read any temp at all or maybe upper or lower limit. Doesn't the obd-ll scanner read the engine temperature through the cpu from the engine coolant temp sensor? I'm wondering if I should replace it? Other info, engine has had intake gasket replaced can tell by newer looking sealant between block and intake. Some other info truck has been inverted(flipped in an accident). At start up it runs smooth. Idles @ 900 rpm and after 15-20 seconds drops to 600 rpm. In order to make it run like new should I start with a new catalytic converter? thanks again for the tips.