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Skyvisions, ASE Certified Technician
Category: Toyota
Satisfied Customers: 14505
Experience:  Toyota Master Diagnostic
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Toyota Corolla: Ive been through 8 emissions tests now and

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I've been through 8 emissions tests now and cannot seem to get the NOx down to the State of Colorado's standards 1.5000. I have a 1998 Toyota Corolla and my last test results were:

HC GPM Reading 0.0749 Limit 1.200
CO. 0.2078. 15.000
NOx. 1.6305. 1.5000. Fail

I have already:

Changed Oil/Filter
Air Filter
Spark Plug/Wires
PVC Valve
Front and Down stream O2 Sensors
New Catalytic Converter
TPS Throttle Sensor
Cleaned throttle housing
Running 91 Octane
Running Lucas Oil in gas to clean jets

If you could make suggestions beyond what I have already done it would be much appreciated. Thank you

Thank you for choosing JustAnswer and allowing me to help you resolve your Toyota questions!

Because this engine does not use an EGR valve everything else has to be 100% and ready to go. From the list of things that you have done the only thing that I can see that could possibly cause the NOX to fail is a week or very poorly operating knock sensor. Knock sensors themselves will usually set a code when they malfunction but this is normally only happens when it is a total failure either open or shorted. If the knock sensor itself just is weak and is not sending the proper signal to the computer it will throw off the ignition timing or not retard for a knock situation. Engine ping or knock is what is contributing to NOX failure. pre-detonation or pink is what drives up the NOX number. I normally never recommend just replacing parts. This being said the only way to effectively test a knock sensor is with an oscilloscope. The knock sensor on this vehicle is located behind the intake manifold on the side of the block it is almost directly behind the intake runner tube going to cylinder number two. Excessive chain play or stretch due to high miles could possibly drive the ignition timing off slightly but that would be a worst-case scenario causing the failure on NOX. Because the HC does not seem to be out of order this only leaves

the knock sensor.
Customer: replied 5 years ago.

I went out to check to see if I could locate it...would it be under manifold cover? Do you have a picture of what it looks like?

Is not under the engine cover it is behind the intake manifold almost directly back from the intake runner tube number two for cylinder number two. The sensor itself is very difficult to see with a little short pigtail wiring harness. Is a single wire and the sensor itself screws into the block and looks very similar to the oil pressure sending unit. Click this link and save for a more specific location. It is very difficult to get to this sensor without pulling the intake manifold.
Customer: replied 5 years ago.

How difficult is this to replace on a scale of 1 to 10? Any idea on how much this will cost if I hire it out or if I do it myself?

On a scale of 1 to 10 it's about six maybe seven. If you remove the intake manifold which takes about an hour it is not that difficult the sensor itself is a piece of cake. I would use only a Toyota knock sensor and typically they average between $180 and $225. Again this is about the only thing that really weighs heavily on the oxides of nitrogen or NOX. Without proper testing I can't say for 100% certainty that this is going to solve your problem. You have ruled out just about everything else provided the compression is good and the injectors are not restricted this is about all you have left. Even with any of those items being off it would affect the HC and CO which you're not having an issue with.
Customer: replied 5 years ago.

One last thing...I don't know if this will narrow it down but after taking my vehicle in today the NOx GPM was 1.0457 (PASS) but the overall test failed due to the gas cap integrity. So I went and purchased a new gas cap and returned to have it tested again and that is when it failed with the NOx 1.6305. On all prior tests the gas cap wasn't even in the equation. Any idea why?

It could be either an intermittent problem with the knock sensor or if you have excessive amount of carbon buildup on the Pistons and the top of the cylinders and the face of the cylinder head this can contribute to high nox. The only way you can clean this effectively is by putting top-end engine cleaner down into each cylinder with the spark plugs out a crank it over by hand until the fluid comes up into the spark plug hole you have to let it set for about 30 min. per hole then lay rags across the top of the engine and crank the engine over to blow all the junk out of the cylinders. You then will have to do an oil change. It's a messy job but it will be carbon the top of the engine better than any junk you put in the tank. Have a ridiculous that the EPA let you pass once then just because the gas cap has you rerun the entire test. Thank you politicians.
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