Hi all, my 1995 LT1 with ~78,000 miles is running rich (very smelly) and throwing a CEL (H64) when idling for more than 2 minutes at temp (CEL disappears when driving). Because of the H64 code, which indicates the bank 2 O2 sensor #1 is reporting a lean condition, I'm guessing the computer is trying to inject more fuel in response to a false lean reading. I'm planning on running Seafoam through the fuel system to clean out some carbon, which probably should be done anyway, then replace the O2 sensor... or sensors?

I know the 1995 models have three O2 sensors, two on the driver's side pre and post-cat (bank 1) and one pre-cat on the passenger side (bank 2), but I do not know which one on the driver's side H64 corresponds to. Also, is it good practice to replace all O2 sensors rather than just one? I've only had this car for a few months and have no clue about its service history. Can anyone tell me which O2 sensor corresponds to the H64 DTC and if I would be better off replacing all of them (and if I'm even approaching this problem correctly)?

Thanks,
Brandon

 

the two in front of the cats are what the computer uses for fuel calculations (lean/rich). Most O2s behind the cats are used to measure the efficiency of the converter and let you know when it is time for that thousand dollar trip to the muffler shop. My personal thoughts on replacing one vs all depends on two questions. How much does it cost and how much of a pain is it to change out. O2 sensors are not that expensive (relative term) but they can be a bear to get out as the heat can really stick them to the pipe. Highly recommend the tool just made for that job.

 

O2 sensor sockets are rentable from most auto stores with a rental program. Not sure if 95 is too different from 94, but the drivers side can get the wrench on it very easily from under the car, and the passenger side's is best gone at from the top - down through the engine bay and right onto the top of it... with a long enough socket extension. If you have a DIY auto shop in your area, access to a lift is what really made it a cinch. For the passenger side, get underneath, stick the O2 wrench on, go on top, hook up the extension, off, on, done.

As for the problem, mine never threw codes but it was the first thing I did when I got the car and they were very, very gone. It was getting a total average of 13mpg after having just driven it home 4hrs on the highway and only a week of in town driving. Nowadays and even before I gave it a new ignition system it's back where it should be around 16 in town.

I want to say the codes don't indicate which side of the car, but if you get under there to do one, you may as well do both.

 

some of the better scanners offer live data mode. This would allow you to see the reading and tell which one is showing out of scale (or nothing). Scanners are handy tools. Or just replace both. But if you are going to live with a C4, scanners are very helpful.

 

Thanks all for the advice- I ended up buying two new GM O2 sensors that I’ll replace in a few weeks once I get through this semester. I tapped the trouble sensor with a multimeter and after letting it get up to operating temp the voltage was really slow to respond to any throttle and seemed to stick around 0.7 mV. I wasn’t able to recreate the CEL condition at idle with the sensor unplugged, so I bet it’s just fouled or something. I did also notice some oil in the connector contacts though… I’ll have to clean that out too. I’ll look at the “good” sensor (driver’s side) later too, hopefully on my laptop once my ALDL cable comes in. I’ll update later on what I find

 

Depending on your multi-meter it may not react fast enough to accurately track all the changes the sensor it putting out. A correctly operating sensor will track up and down as adjustments are made by the ECM.

 

The spec sheet for my multimeter lists a 2 Hz sampling frequency so maybe that's not fast enough

 

Update on this thread: I got a diagnostic cable and started data logging using EEHack. After driving around for a while, I was able to recreate the DTC #64 when idling for around a minute. The top graph in the screenshot shows the right O2 sensor voltage (red) and the engine RPM (blue) overlayed. The green window shows when the DTC #64 appears. On the car's driver info screen, the CEL disappears as an intermittent, but the datalogging software doesn't show this. Instead, look at the bottom graph's red window which shows when the car goes back into closed loop, which happens when the CEL disappears on the info screen. Also on the bottom graph is the left O2 sensor voltage (green), which shows normal operation. What's really confusing me is the right O2 sensor voltage dip beginning around 1400 sec. I have no idea what might cause this, and why it doesn't return to normal fluctuation after the CEL disappears and the car goes back into closed loop. Is this just typical behavior from a bad O2 sensor? If anyone wants to take a peek at the log file, I can send it through email or something. ALSO- just so no one is confused, there was a pause in data logging around the 800 sec mark.

 

Last Update: I finally got around to replacing both O2 sensors, and now there is no CEL, and both sensor voltages fluctuate rapidly like they should! When pulling the old sensors, the one reporting the lean condition was covered in soot (no surprise because it was running rich on this bank) and the seemingly okay one looked pretty destroyed by silicon poisoning. The worst thing though was that the sensors were actually different models... thanks to one of the 8 previous owners for doing that. I also cleaned out the O2 sensor contacts with some electronic cleaner spray just to get rid of any oil/debris.