1995 240sx: DTC #33 Can only the dealer read all diagnostice info?

[KRUZNBY]

OK, I have a 1995 240sx OBDI that is throwing code #33 (Front Oxygen Sensor). About 4 months ago it was throwing the same code, so I replaced the fuel filter, added a can of BG44 and finally replaced the front O2 Sensor and all was fine until now I have the same code.

Symptoms:Gas milage has not changed noticably. Occasionally, the car will hesitate for almost a full second, but then be fine for the next half hour or longer. This hesitation is not like when you you have no spark on one of your plugs since it lasts too long.

Question:Would only the dealer have the ability to use a scan tool to read the advanced diagnostics such as voltages from the different sensors to determine if it is running too lean or rich etc.? Or, is this common place and a lot of 3rd party garages could do this? I really don't want to pay a premium to go to the dealer.
Modified by KRUZNBY at 2:33 PM 7/15/2006

[s13sr20chris]

some indy shops can look at it but not all. did you use one of those cheapy universal o2 sensors?

[KRUZNBY]

No, I bought an OEM O2 sensor. I figured at the very least, it was preventative maintanence.

HaHa "Indy" Good thing I've watched Entourage.

[NISTECH]

If the shop has a snap on scan tool with all the software cards they can moniter most of the sensors. Also a DSO's are becoming pretty common place in the industry now. If they have those 2 tools they are more then capable of diagnosing your car. My guess is [ assuming the new o2 is still good you have a wire or connector issue somewhere.

[KRUZNBY]

OK, it's been about a year now and I still have this problem. I took it to a shop and they said the O2 sensor was reading a negative voltage and replaced it. About 1 month later the light came on again and they said the sensor was not working again, so they replaced it with an OEM one. Two weeks later it was throwing the same code. This time I take it back and they say the O2 sensor is reporting the correct voltage. They tested the ECU wiring harness and said it was fine. The recommended changing the ECU. Today I put in a used ECU and for the first 20 minutes, I didn't noticed the problem at all. I turned the car off and right back on and drove less than .5 miles and the light came on again.

The car idles fine. If I'm driving and go from no throttle to a very small amount of throttle, the car often hesitates and bucks back and forth. If I give it more throttle, all is fine. If I drive it like a stole it, I would probably not have any problems.

What could set the O2 sensor off? I was thinking I might have a bad injector, but wouldn't idle rough too?

Any advise would be greatly appretiated.

[lbc240]

you might want to check the ground for the sensor. i've seen bad or weak grounds cause a lot of intermitent problems and are hard to diagnose.

[KRUZNBY]

Update:

Today I decided to hook up a voltmeter to the ECU harness and monitor the voltages for the O2 sensor while idling and driving. As expected from a cold start and idling ~2000 RPM (I'm guessing since my tach is not working either) as in the FSM, I got voltages between .1 and .9V. Giving more throttle increased the voltage and letting off the throttle decreased the voltages momentarily.

For fun, I take it for a test drive with the voltmeter hooked up. I drive towards the Interstate and when I let off the throttle sharply, I see negative voltages. After WOT I let off and the voltage goes to -.75V and for the remainder of the trip, the voltage remains negative. Even while negative, the voltage fluctuates between .-8 and -.01V. At WOT it approaches 0V instead of 1V as expected. When I shut the car off, it reads -.75V and slowly reaches 0V over time.

I tried both ECU's I have and they both give the same symptoms.

What would cause negative voltages?

Again, I've replaced the o2 sensor about 4 times now, so I doubt that is not the primary problem. I have not replaced the O2 sensor lately, so I guess it is possible it is fouled in some way.

Any suggestions would be greatly appretiated.

[NISTECH]

The readings your getting is called phase shifted. This is usually caused by a bad O2 sensor. Where are you tapped into the O2? And if its in the engine bay is it disconnected while conducting the test? It wont fluctuate as much unplugged but you should not see a neg voltage from it.

Also before just throwing another o2 at it, when the problem occur again and your seeing the negetive voltage take a look at you ground to the exhaust manifold from the neg battery terminal , see if you have a good ground to your exhuast.

[KRUZNBY]

I was measuring the voltage at the ECU harness between pin 46 and the ground pin 43 while the O2 sensor was installed normally.

I've never used a voltmeter before this experience. I saw this thread in the Articles section zerothread/159907 and checked the ground based on other suggestions here. I measured close to zero ohems from the negative battery terminal to a screw on the fender. Should I do the same thing, but check it to the exhaust instead?

[NISTECH]

yes you want to check between the battery and the exhuast manifold.

Also try testing your o2 when its failing with it disconnected at the connector off its pigtail. The sensor is voltage generating. Each test is checking for a different fault but do both tests.

[KRUZNBY]

OK, I checked the ground from the battery to the manifold and there was good connectivity. I then tried checking the O2 sensor at the sensor with a voltage meter. I turned the car on and grounded the voltmeter to a fender bolt and checked with the sensor still connected. I got .13V. More throttle gave a max of .2V and a sharp let off of the throttle it was near -.05V. I let it warm up for ~5min and checked again. .16V at idle and revving did not increase much. Sharply letting off the throttle yeilded -.5V.

I just realized that you suggested trying it with the sensor disconnected. Unfortunatly I have not tried that. I'm guessing my O2 sensor is shot, but there must be a more serious problem upstream that keeps fouling my O2 sensors.

I disconnected each spark plug wire one by one and noticed a rough idle, so I must be firing on all cyliders. The other options as suggested by the FSM is injectors and low fuel pressure. First off is there a way to check my injectors?

Any other suggestions?

Thanks for your help.

[NISTECH]

the negetive voltage reading indicates the o2 is bad but you must verify this with the o2 unplugged. If all readings seam normal with it unplugged you may have a short in your wiring from the signal line to the braided sheild wrapped around it.