1995 blazer 2dr 4.3L. 4wd. How many sensors does it have and what brand is recommended either to use or stay away from. The sensors on the engine side of the cat can affect gas mileage, is that true ?

 

The early production 4.3L has 2 sensors, late production has 3. The sensor(s) that are in front of the catalytic converter are used by the PCM for monitoring and controlling fuel mixture. Sensors behind the converter(s) monitor the efficiency of the converter(s) and have no affect on engine performance or fuel mileage. The sensor(s) react to changes in the oxygen content of the exhaust. The faster they react, the better the fuel mileage. During the life of the sensor, the reaction time slows down. Around 100K miles the reaction time has slowed enough to substantially affect fuel mileage and need to be replaced. AC Delco and Denso seem to have the fastest reaction time, with Bosch in 3rd place

 

is 2000 considered late production? i have no catalytic convertor on mine but i also do not have a SES light on my dash. there is just straight pipe welded where the cat was but has a poor welding job from PO. i was gonna put a new pipe on it, but i didnt wanna screw with it if its gonna give me SES light (i hate seeing idiot lights #1 pet peeve with cars). if i have any sensors after the catalytic convertor i should have a check engine light right?

 

The "late production" referred specifically to the OP's 1995. Your 2000 originally had 3 oxygen sensors: Two in the "Y" pipe, (one near each exhaust manifold), and the third sensor should be downstream of the catalytic converter. If the converter and/or sensors are missing, the SES light should definitely come on. Originally the catalytic converter was part of the "Y" pipe, but an aftermarket universal can be welded in. If the bung for the oxygen sensor is missing, they are also available aftermarket.

 

i can definitely confirm i have no catalytic convertor and a slightly smaller diameter pipe was poorly welded. it is leaking exhaust around the welds. but i can also confirm i have no SES light. it goes on with the key then goes off, so i dont know whats up. what would you recommend i do? i found all 3 sensors with 3rd being right after where cat used to be. if i tried to replace the pipe im afraid i would trip something and throw the light

 

If all 3 sensors are where they should be, and are they all connected, and the light is off, the monitor for the sensors may not have run and completed its test, or, the PCM may have been flashed or a PCM tuner may have been installed.

 

is there any way to tell if the pcm has been flashed/tuned? im also reading about an antifoul trick for the o2 sensors? like a spacer or something?

 

First check the status of the monitors with a scan tool. Make sure they have all completed their tests. If the oxygen sensor tests are not complete, the PCM will not turn the light on or set a DTC. As for the PCM, it would be best to flash it to the VIN, then you know for sure that it's correct, you'll need a dealer to do it though.

 

ok it really doesn't matter if i have the cat or not (i live in michigan, no emission tests) all i want to do is basically fix the crappy weld job the PO did when he welded the straight pipe in and get rid of my wicked exhaust leak, i just dont want to go through the trouble of fixing it to discover i would then have SES light.

 

It's against the law, (Federal) to tamper with any of the emission control devices in all 50 States. Just because there is no emission testing, doesn't mean that you won't get caught, (there are other agencies that can and do enforce the law). $3500 fine.