95 240sx Code 77 Please Help

[jg61022]

Hi. I have a 95 240sx. The ECU is throwing a code 77. I replaced the rear O2 sensor, and it didn’t help. Would an aftermarket air intake cause this problem? Any help would be greatly appreciated. Thanks.

[JeromeS13]

Have you followed the steps from the FSM, starting at page EC-162?

-Jerome

[jg61022]

Hi. I haven't try it yet. I also did a smog check. CO and NO are low, HC is high. According to EC-162, code 77 could also mean bad fuel pressure regulator. Could that be it? I don't see any fuel leak. The only mod I have is an aftermarket intake. Thank you very much.

[JeromeS13]

It also mentions a air intake leak, right?

[NISTECH]

What were your emissions readings? If you had a vaccum leak your nox should be high too.

[jg61022]

Hi. Here are the emission readings. Thank you very much for everyone’s helps.

HC at 15 mph MAX ALLOWED: 88, MEASURED: 86 HC at 25 mph MAX ALLOWED: 53, MEASURED: 53

CO at 15 mph MAX ALLOWED: 0.52, MEASURED: 0.26CO at 25 mph MAX ALLOWED: 0.50, MEASURED: 0.16

NO at 15 mph MAX ALLOWED: 704, MEASURED: 116NO at 25 mph MAX ALLOWED: 738, MEASURED: 72

[NISTECH]

It looks like you passed the test but your hc and co are above average for your vehicle. The rear o2 has no effect on fuel mixture it is simply used for cat monitering. You are either getting too low or too high of voltage going back to ecu or the duriation of time it is in one side or the other of .55v is too long. This is a 2.2volt ranging sensor not your typical 1v sensor. You need to break out a multimeter and test the voltage getting back to terminal 52 of the ECM. Rig it up so you can watch your voltmeter while driving. What you will be looking for is the voltage to drop close to 0v on a long deceleration and for it to go over .7volts on a long hard acceleration. If its not doing that it indicates the sensor is not functioning properly or you have a wire issue. Another thing is it needs to be able to fluctuate from above to below .6 volts. If this sensor is stuck reading really high or really low then you dont have an issue with the 02 but rather an issue with your fuel mixture. Seeing your emissions readings I dont really suspect that to be the problem. Something it doesnt mention in the service manual is exhuast leaks. if you have an exhuast leak it will draw in whats called pirate air. Air that may not have been seen by the front o2 but gets picked up by the rear o2. This will cause the o2 to send an oxygen indication to the ecm that is not with in the range the ECM expects to see, as compared to front o2 readings.

[jg61022]

Hi. Thanks for your help. I do have a cheap aftermarket intake I got on ebay, and I have replaced the EGR pipe. I wonder if there is a leak at the EGR pipe or intake. If it does, I guess it will cause high HC, CO, and code77. I will check it out. Thank you very much for everyone’s helps.

[jg61022]

Hi. Sorry if this question sounds stupid. How do you check for intake vacuum leaks?

[jg61022]

According to my emission result, is the fuel mixture too rich? Thanks

[NISTECH]

Your not overly rich, however if you are getting pirate air in the exhuast near the front o2 it will interpit that as oxygen coming through the engine, the ecm will then increase injector pulse width to compensate for the percived lean condition.

[jg61022]

Hi. I just took my 95 240sx to a Nissan dealer. They said the voltage signal from the rear O2 is too high, 2.55V, which causes the error code 77. I have just replaced the rear O2. What can cause a high voltage signal from rear O2? It's difficult for me to get underneath the car, so I can’t perform the EC-165 test myself. Does anyone know a reliable Nissan mechanic who can perform the test? I live in San Diego, CA. The dealer did not do the EC-165 test for the one hour of diagnostic I paid, they want more money for it.

Modified by jg61022 at 6:05 PM 6/3/2006

Modified by jg61022 at 6:06 PM 6/3/2006

Modified by jg61022 at 6:42 PM 6/3/2006
Modified by jg61022 at 10:04 PM 6/3/2006

[NISTECH]

2.55v is the maximum amount of voltage the ecm will show on that circuit with consult. My guess is you have a short in your wiring harness from the heater circuit to the signal circuit. Or you have a burnt driver in your ecm. What you need to do is test for voltage on the signal wire for the rear o2. I bet it reads close to 12volts even though consult is showing 2.55v. This is something they should have found in 1 hour of diagnosis. I am a bit confused at what they did for an hour. Its a simple circuit with 3 or 4 wires. You test it plugged in and test it unplugged. If the high voltage goes away when you unplug it you have a bad o2. If it doesnt go away you have a short in your wiring or a fried ECM.

[jg61022]

Thank you very much for your helps. I really appreciate it. You have helped me a lot more than the Nissan dealer did. Thank you. I am going to take it to a different shop and have them check the voltage of the rear O2 signal wire. I am also thinking about purchasing a rear O2 simulator from Ebay to by pass the rear O2.

[jg61022]

Hi. I am following procedure EC-164 to check the voltage signal from rear O2 to the ECM. The followings are the result. Do you think I have wiring problem?

I used the multi-meter and checked the voltage between ECM pin 52 (signal input) and ground while the engine is running, I got a 0 volt.

I checked the voltage between ECM pin 108 and ground, while the engine is running, and while the key is at the on position, I got a voltage of 12 volt both times.

I though the wire to pin 108 is not suppose to send voltage to ECM. Am I correct? Do you think this is wiring problem? Thank you.