Failed smog...need help

[rod_b]

EGR not connected, failed visual inspection. Is this something the "mechanically challenged" like myself can fix? Also, rear bank O2 sensor throwing errors. Is it related to the EGR problem?

[XPLORx4]

EGR not connected, failed visual inspection. Is this something the "mechanically challenged" like myself can fix? Also, rear bank O2 sensor throwing errors. Is it related to the EGR problem?

 

Did you go to a test-only facility? If not, the shop that performed the emissions test can fix the problems.

 

The engine emissions configuration on your '96 may be different than my '97, but you can start by downloading and reading this PDF of the engine control systems, including the EGR system. EGR function description starts on page 361 [EC-209].

 

The EGR and EGR-BPT valve are attached to the right side of the intake manifold, as you face the engine. (See attached photo.) Vacuum hoses connect to the valves. It's possible that one or more of the vacuum hoses are disconnected, which would cause the visual inspection to fail. There should also be a vacuum hose diagram on the underside of the hood that can help you find the disconnected lines.

 

The rear O2 sensor error can be likely fixed by replacing the rear O2 sensors. They are located on the underside of the vehicle, attached to the exhaust pipes.

 

The rear O2 sensor is behind the metal exhaust shield that's sticking out, to the left of the light in the photo below. There's a matching one on the other exhaust pipe.

 

[rod_b]

Thanks XPLORx4.

 

I went to a test only facility. The back hose of the EGR-BPT was disconnected so I did get that back on. Is replacing the O2 sensor as easy as screwing it in and clipping in the wire harness? Do you think the O2 sensor throwing codes or possibly even being bad cause the high emissions?

 

Thanks for your help.

Edited February 20, 2008 by rod_b

[XPLORx4]

The rear O2 sensor failures don't necessarily cause high emissions. Which code(s) do you have?

P0136 and P0156 are for the left rear O2 sensor, and P0141 and P0161 are for the right rear sensor. If you have a code P0420 or P0430, then it means that the rear O2 sensors are detecting that the cat's are no longer doing their job, and are bad.

 

Did the tech even do a gas reading test on your truck? If so, what were the gas (HC, NOX, CO) readings from your smog test?

[rod_b]

Yep, P0156. Should I just replace it? The thing is, I cleared the code once at AutoZone with their computer. It came back on after about 300 miles. Then went off after another 100 miles. Then just came back on. Probably needs to be replaced. Yes, they did do a gas test. HC and CO well within passing values. NOx was bad...

 

15MPH

MAX-MEAS

508-864

 

25MPH

MAX-MEAS

761-813

 

I have not a clue where to start to try to get these numbers down. Thanks for your help.

 

Rod

[XPLORx4]

Yep, just replace the sensor. It's in its death throes. As for the high NOx readings, that's due to the EGR system being disconnected. The low HC and CO readings are good news that nothing further is wrong. Also, note that after resetting the computer, all of the OBDII 'readiness monitors' are reset as well. There are 6 readiness monitors, and at least 3 of them have to indicate "ready" in order to pass the SMOG test. It will take a few "drive cycles" for the monitors to be set, so don't reset the computer then right away get retested.

 

Click here for an example drive cycle that should set all of the monitors. There is a series of Nissan-specific driving patterns in the FSM that you can follow as well.

Edited February 21, 2008 by XPLORx4