92 740 2.3 non-turbo with Regina failed emissions on CO and NOX. May have been the Marvel Mystery oil added to fuel and not giving it a good drive before test. Original cat and O2 sensor with 166K+. Have done several things but feel that should replace both before retest. And reading about replacement sensor, is NGK 25002 right one or 25003? Will appreciate any help.

The FAQ says 25002 although some auto parts places will errantly say a universal will work (it won't). The test procedure described in the 700/900 FAQ works well for testing your sensor. Pulling the vacuum off the FPR worked great for me to induce a rich condition and then pulling the vacuum to the flame trap worked for inducing a lean.

Ron
93 245
93 940

I wouldn't think it was a good idea to do an emissions test with Marvel Mystery oil added to the gas tank. Maybe try it again after a couple tankfulls of plan gas. Or you could give the Marvel folks a call to see if they have any info regarding the effects of their product on engine emmisions.

If you post what your readings were (and the allowables), that would help us with a diagnosis.

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In God We Trust. Everyone else, keep your hands where I can see them.

HC reading 1.93 Limit 2.25
CO 29.16 23.74
NOX 4.84 3.25
CO2 411.86

The high CO says you're running rich. Within limits that shouldn't drive the NOx up, but if you've been running rich for a while the cat may have lost its effectiveness.

O2 sensor is the right place to look; per another post, try to get a voltage reading with a decent DMM and post back with your findings.

If the O2 sensor turns out to be bad, I would disconnect the battery for 10 minutes to reset the ECU 'memory' (not sure if your ECU is 'adaptive', but this won't hurt if it isn't). Then take a few long drives (1 hour +) to get the cat up to operating temps for a good length of time for a few cycles, hopefully to burn off any light contamination. Then if your emissions still test high, it's probably time for a new cat.
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In God We Trust. Everyone else, keep your hands where I can see them.

Smitty, see you are online now. Just failed test second time after doing all I could. So it's time to replace the original cat. Hope the good O2 sensor comes out easy enough. Shopping for good direct fit OE one online. I've read that Bosal and Carsound are good choices. Any recs from you? Will be online for next half hour or so, then I'll be deciding.

Smitty, thanks for comments. Don't hv much time right now. Found where connections are under brace near right hood hinge. Plan to test there for output voltage. Will ck 700/900 FAQ, thanks to other responder. Also what is best manual to use for my car? Hv Bentley 240.

Suggest you diagnose before throwing parts at it, that is expensive and you will only fix it if you guess correctly. As a start:
Check engine light on?
Is there a stored fault code?
Measure the O2 sensor output voltage.
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David Hunter

Just ckd O2 sensor output voltage and it's doing what it should varying between less than 0.2 and more than 0.7. So you all have just saved me over $100. Thanks. Next want to ck MAP sensor output and any other suggestions you all may have.

No ck eng light on and no stored fault code(got 1-1-1 flashes)
Have not ckd O2 sensor output voltage. Will work on that and get back to you. Thanks, David.

99% certain it is the cat, at that age & mileage. If you correct the high CO by running leaner, (say by changing the O2 sensor or whatever) then the NOX will go up. A new cat is only about $150 or so if you search the web. Also tests are not free. CO2 - should this not be several % not 400 ppm?