And I'm PISSED! It'* a 97 Bonneville SSEi with 150,000 miles

The emissions test is done while at idle as well at around 1400 rpm, on a dyno and with the tailpipe sensors in both tailpipes. It barely passed in the curb idle test but not the ASM =2525 test which is done at around 1400 rpm,

These were the readings

ASM2525 Test

HC ppm - Limit 55 - Reading 69 ......FAIL
Co% - Limit 0.31 - Reading 0.41....FAIL
NO ppm - Limit 408 - Reading 1245 ..FAIL

This is what I've done to prepare for the test and the car has 150,000 miles.

Changed engine oil and filter two weeks ago.
Changed air filter today.
Use high octane fuel
Change plugs (NGK tr 55) plugs 7 months ago.
Cleaned exhaust tips of excess carbon build-up
Check that there were no codes stored
Ran car on highway for 10 minutes before test
Coolant was at 195 degree as seen via scan gauge


Two years ago I changed the complete exhaust including a high flow cat, new Belden wires and tune up, along with oil change and filter and cleaned EGR Valve and it tested as follows with a pass.:

ASM2525 Test

HC ppm - Limit 55 - Reading 17
Co% - Limit 0.31 - Reading 0.05
NO ppm - Limit 408 - Reading 368

What the hell!!!!

What high flow cat did you use? A lot of those don't last very long and definitely don't test very well. I know I've heard about a lot of Magnaflow high flow cats that after a year simply don't do that great of a job anymore.

Magnaflow high flow cat. I can understand it causing a bit of a poor reading since it'* two years old but to fail on all three tests due to the cat I find diffcult to grasp.

I honestly don't know a whole lot about cats and what they help correct or reduce as far as emissions testing goes. What I do know is that magnaflow cats don't seem to last many people more than a year and they stop doing their jobs properly. There'* a reason why you can get them for as low as $50 a pop brand new on ebay and factory replacements are $200+. Last year I paid $100 shipped for two of them and they were in their original packaging.

1 Bottle of Gaurenteed to Pass (I used it before my last emissions test)

I'm assuming you have changed the front O2 sensor, at least 2-3 times in 150K. The O2 sensor basically controls the scrubbing of the HC and NO. The CO is just a general byproduct of the other 2 being scrubbed.

If you have a scan gauge, look at the reading of the front O2 sensor. It should be switching from Rich to Lean, or from low numbers like 1 to high like 9.

It should be doing it faster than you can read it.

If it is, then your cat (or maybe your EGR, but you would usually get a code) is a dead monkey.

If it is moving slow enough that you can read the 2 decimal places in the number, or it is switching in a range of say 4 to 7, or just plain erratically, then the O2 sensor is the culprit.

Meanwhile, if the O2 is bad, the computer sees this, particularly if it is hanging on the Lean side,... and no longer gives a chit about emissions. It will command air, fuel, and spark mix to get you home safely without chipping a piston. In other words, it will bury your performance and make the emissions even worse. Which, if you run long enough like this, you could bury your cat in carbon. Or you could get the dreaded P0171.

That is the only error code in the FSM, which troubleshooting section begins.....

"If you have considered killing yourself in the past 6 months, it will save your family and friends a lot of grief, if you just put down the manual and step away from the car. Braver men than you have tried and failed."

Humor me, and check the front O2 sensor, please.

Any idea what'* recommended long term other the then original?
I did a search on G. T. P and it seems it helps improve somewhat but one when you're almost there. There have been as many good review as there are bad so I can't rely on it but I guess it'* worth a try, although I have to pay each time I retest so I'll like to try a couple more suggestions as well.
The O2 sensor was changes approximately 20,000 miles ago with the A/C Delco ones. Since there are no codes stored I assumed it was doing a good enough job to atleast give me a reading enough to pass emissions. There is a Aeroforce scan gauge Mick installed years ago, with this give me the correct readings to test the 02 sensor?

Thanks for the responses everyone.

Try this. Pull your spark plugs and inspect them. Are they dark or black? If so your running rich. If your running rich you over loaded the O2 sensor, it'* shot, and you over loaded the cat, it may be shot.

That'* excellent. DELCO or Anso are best bets. It only takes a minute to check and it "controls" so much of what the computer does with air, spark, fuel. If I had a car which suddenly chit the bed on emissions, I would read that first.

You're actually more likely to get a formal code on the rear 02 sensor. That'* what reads the amounts for the emissions test, it'* in line after the cat. If the slightest thing is wrong with the circuit or the sensor, you get a code. Probably engineered that way as an EPA standard.

Your front 02 sensor can fail incrementally, and still go on. . That'* why best practices has always been to change it, when ya first get the car and do the tuneup. The small vents on the sensor metal bulb can get clogged, or the internal sensor can get partially wounded. Where it'* located, I'm surprised they don't screw up every 100 miles. They are one of the wonders of Bonnevillism.

The Aeroforce is probably the best gauge we can get, short of a tuner. (Scangage II isn't bad if you are more interested in daily general operations, and not performance.)

There are 2 O2 sensors and two readings. You want the "01" reading, the front sensor on the rear exh manifold. The reading acronym will say something like "O2S01".

I don't have an Aeroforce.. the reading will either be toggling between "Rich" and Lean", or toggling between low numbers like .75 or 1.25 and high numbers up around 8.00 or 9.0.

If it is working correctly the toggling will be extremely fast. And if it is a numeric display, it must be toggling a WIDE range. Like between 1,2 and 8,9 (roughly). If it is slow , or erratic, at least part of your problem is the sensor.

If it'* working OK, you can move on to hating your cat. Or start looking at fuel trims and ignition advance to figure out if and why your engine is taking a dump on your emissions system.

Spark plugs were still fairly clean considering they were changed earlier this year.

As for the aeroforce, to be honest, Mick installed it and I don't really know what the perameters should read so it'* not much use to me since I don't know what to look for.