I have 2 o2 sensors. Upstream and one by the cat. Are they the same part number? 234-4012 Denso

 

Yes it should be.I have 2 upstream(only) O2 sensors, their both the same look and part numbers.G.M. wouldn`t have made 2 different style`s of sensor`s just for one vehicle.That would increase the cost to produce.

 

There is only one upstream sensor on the 95 and earlier as the CPI engine used a single batch fired fuel injector with no way of independently controlling the banks like the 96+ SCFI engines.

I seem to remember that there could possibly be two downstream sensors depending on what PCM setup the vehicle had; 95 was a VERY screwy year. Upstream sensors are used for fuel distribution controls while downstream sensors are catalyst monitors only. If there were two downstream sensors, one would be directly ahead of the catalytic converter with the other directly behind. The upstream sensor will be the one located closest to the engine and on the CPI engines should be right at the 'y' in the y-pipe. If you count only two sensors from the exhaust manifolds back to the muffler, then you do not have a pre-cat downstream sensor.

Downstream sensors should be replaced only when there is a code for that sensor and diagnostics determine the sensor is at fault. They do nothing for engine performance.

The sensors are the same part number for upstream and downstream. AC Delco or Delphi are all that I can recommend. Years ago I tested the responsiveness of quite a few different O2 sensors and the AC Delco & Delphi sensors had the highest activity with NGK (now NTK for these sensors) not too far behind, but others like Bosch, SMP (Standard Motor Products), etc were considerably lower in response time. Activity directly correlates to life.

 

Okay thanks.. Yeah i looked all up and down the exhaust. I only seen One o2 sensor on the passenger side in the y pipe and 1 other one in the exhaust by the Cat. So I only see 2. I changed it because the truck runs great for 5 min until it warms up than starts to surge when you hold the gas down say at 1500 rpm. than it will go up and down like 3-500 rpms. Also starts to run real rich like eyes start to burn rich.
I've changed the plugs to ac delco factory gap
New ac delco spider injector
New ac delco Nut Kit
New ERG valve
New temp sending unit.
changed the Map sensor also idle control sensor.
I just noticed the other day when I shut it off I heard a small high pitch sound coming from the back by the distributor. When i unhook the vac hose to brake booster the idle hardly changes. I'm thinking the lower intake may be leaking air maybe?