Wanted to tap the wisdom of the forum with this issue. Recently replaced the original spark plugs on my 96 LT4. It is low mileage, 14K, but I thought given the age should go ahead and put in new plugs. Car was running fine prior to this; although had a recent ‘service engine soon’ light come on but that was due to a stuck open tstat that I replaced when I got the car in May, when I changed all the fluids. Put in a new AC Delco stat, and while in that area, put in a new coolant temp sensor (the one on the front of the engine) as well. After that, it was running just fine.

This past weekend, I replaced the spark plugs and that coolant sensor on the passenger side of the engine. Immediately got the ‘service engine soon’ light on again. Took it to O’reilly’s and had them run the codes and they came back as:

P0125 – insufficient coolant temp for closed loop fuel control

P0151 – O2 circuit low voltage (bank 2, sensor 1)

The P0125 was the same code for the first episode of the service engine soon light. Once I replaced the tstat, the coolant temp rose to normal operating temp and the light went away. I assume it was more the tstat vs the new coolant temp sensor as prior to swapping the tstat, the coolant temp would barely get above 150 degrees. That said, the car ran fine, couldn’t tell there was an issue except for the light and the coolant temp not getting over 150ish temp.

The P0151, as I’ve researched, could be multiple issues: could be a bad O2 itself, could be the wiring to it, could be an improperly reading coolant temp sensor, or a failure in the O2 sensor’s heater circuit.

I have checked the wiring to it and it appears fine. I guess the part itself could be bad, but it was working fine before I put in new spark plugs and the new sensor on the side of the engine, in between plugs #6 and #8. I don’t know how to check/test for a failure in the 02 sensor’s heater circuit.

There is a noticeable difference in how the engine runs. It idles a bit differently (rough at times, not constantly) and it produces more exhaust than it did beforehand. Which in my research was symptoms of what I’ve noted about the bad 02 sensor or a bad coolant temp sensor. I have parked it and am not driving it till I get this fixed.

So, I guess my question is, what would you start with first to address this? Should I replace the new coolant temp sensor on the passenger side of the engine with the original one I removed (I did save/keep it)? Or, should I replace the bank 2 sensor 1 oxygen sensor? I’ll have to drop the exhaust to do that and was hoping to avoid that.
Thanks in advance for feedback and suggestions.

 

I don't have a '96, but I do know that today's aftermarket sensors for these cars are a pot luck kind of deal. Many times they're no good right out of the box.

 

Work the coolant temp sensor first.

 

Instead of guessing with money. You need a scanner to see if O2 is active. If you have the trouble tree or wiring diagram; you could check connector for 12v and 4.5v and ground on the other 2. Make sure harness isn't contacting exhaust. If that's good, I would replace HO2 Sensor.

Coolant Sensor works since the slow to heat up fast enough code cleared.

 

Started in on replacing the the front 2 O2 sensors. Just decided to go ahead and drop the exhaust and get it done so I could get the car back on the road. Got into it, and it’s such an ordeal (in my opinion) to drop this exhaust that I decided while I had it out, I would order 2 more O2 sensors and replace all 4 of them. Also, while I had the passenger side cat down and outta the way, I’d order a new fuel filter and replace it (I shoulda done that the first time I had the exhaust out right after I got the car and when I replaced all the fluids; took it out to paint the tips as they were a bit rusty). Anywho, looks like it’ll be on the rack another week because the other two sensors and the fuel filter arrive Thurs or Fri.

Check these pics out….no wonder that bank 2 sensor 1 was throwing the SES light, just look at it.


All 4 OEM oxygen sensors. Bank 1, sensors 1 and 2 on left; bank 2 sensors on right.


OEM Bank 2 sensor 1 on left; new A/C Delco sensor on right. No wonder that old one was throwing the error code causing the SES light to come on…..


Bank 1 sensor 1, new one, installed. Actually was able to replace this one without removing the driver side cat.


Ordered new Delco fuel filter, will replace while exhaust down and out of the way.

 

Parts came in Thursday. Yesterday, installed new fuel filter and remaining O2 sensors, then buttoned her back up. Popped into O’Reillys today and erased the code and she’s running like a top now! Ran some errands, no SES light, all good.

Next project, after a break from working on her, is new intake and valve cover gaskets.


New fuel filter.


New O2 sensors, bank 2.


New O2 sensor bank 1, sensor 2.


New O2 sensor bank 1, sensor 1.