My car failed on Nox, everything is up to snuff, no egr problems, new convertor, etc.

Transmission switch for failed Nox?

When folks talk about a trans switch being the problem, exactly which switch are they talking about?

87 auto

I may be thinking of a EGR switch.

Thanks Greg

 

Excessive Oxides of Nitrogen (NOx):

High NOx can occur when there is excessive temperature in the combustion chamber or a damaged catalytic converter.
Check the following:
• Air Injection System
• Exhaust Gas Recirculation System (EGR)
• Combustion Chamber Deposits
• Oxygen Sensor
• Catalytic Converter

Diagnostics for NOx failures

OVERALL
·Check for causes of high combustion temperatures. Primary systems that affect NOx include:
-mixture control (lean)
-EGR
-ignition
·Check Catalytic Converter

1981-86
Check the following:
O2 sensor (The signal from the O2 Sensor provides a wealth of information on how the vehicle is running.)
EGR valve and EGR hose
Ignition timing and system (advanced timing and ignition components are some of the most common problems)
Vacuum lines (deterioration of vacuum lines, inlet manifold seals become more prevalent causes with age)
Catalytic converter
Primary feedback sensor failure, including oxygen sensor, MAP/MAF, TPS, RPM, and CTS sensors

1987-95
Check the following:
Use OBD, if available
O2 sensor
EGR valve and EGR hose
Ignition timing and system (advanced timing and ignition components are some of the most common problems)
Primary feedback sensor failure including, oxygen sensor, MAP/MAF, TPS, RPM, and CTS sensors
Vacuum lines
Catalytic converter

 

It is a 87, fresh rebuild on heads, all of the rubber hose parts around the intake have been replaced, correct factory egr valve and temp sensor 1 year old.

We are getting a good swing on the O2 sensor and the wide band shows 14.5 to one at cruise.

All of the passage ways for the EGR are clean, very clean.

it has a new direct replacment cat on it.
Ignition timing is at 5 BTDC.

the coolant temp sensor is less than six months old.

With it hooked up to a Snap-On Modis, the CTS, O2, tps and mass air flow all look very good and they graph out with no glitches.
We have done this in "generic" so we know we are getting actual values.

I had this problem last year and had to hook a water bottle up and retard the ignition advance to make it pass.

It has set no codes in the last year, other than induced codes.

A couple of things make me wonder, the CO and the HC both are at zero, which indicates a lean mixture and that leads to higher cumbustion chamber temps and higher Nox, but the wide band does not show a lean mixture.

My guess is that the new cat is really doing its job of cleaning up CO and HC's.


The Nox is higher after this last valve job, which makes three head jobs on this motor, I know for sure that the head has been cut three times which along with a .60 over bore will raise Cr.

Other than the blower this motor is stock rebuild and has passed for several years.

I am a pro mechanic, just not so much on Chevys of this vintage, if it was made in Europe I might have a better handle on it.