1999Porsche911

Cruising for extensive time in your car will not damage your cats. Assuming your O2 sensors are working within specs, you are running lean while cruising. I frequently cruise at 3000+ rpm for 18+ hours straight and have never experienced problems with cat damage. As a matter of fact, the Motronic's computer requires numerous different engine speeds below 3000 rpm to even calculate a faulty oxygen storage capacity of the CAT.

Tippy

I have had my CEL on and off for 22,000 miles, I dont pay attention to it. If its hitting on all 6, runs smooth, has good oil pressure, doesnt run hot, and gets good gas mileage, I dont pay attention to CELs. If a sensor gets out of its parameter 1 time, its triggers it. I only care about the vitals, which the CEL cant tell me.

NNH

Thanks for all the input so far. The idea about dead O2 sensors is a good one, but I think the original problem was really with the left cat. The reason is that the guys at AMS hooked up their laptop before I went back to Porsche, and they showed me that the post-cat sensor voltage was swinging up and down on Cat 1, whereas on Cat 2 it was less volatile (although they said it was still swinging a fair amount and Cat 2 might be dying too).

I'll hand it over to the dealer on Tuesday in the hope that I'll get a shiny new passenger-side catalyst out of it, and if it turns out to be a different issue then I'll reconsider in light of all the advice here.

The car certainly feels fine, and I can't detect any loss of power, gas mileage or smoothness, though I think the rate of oil consumption may be very slightly higher.

NNH

Latest update:
The code is P0430, which I think is associated with the passenger side cat. The car is back in the dealers' hands, and today's courtesy car is a Subaru, which is certainly an improvement on the Fords I've been given before.

stilton_a_cheese

NNH:*but in town I tend to surf the torque curve at 2000-3000rpm. I've noticed that a lot of other rennlisters claim to hit 4000+rpm in each gear, but I must confess that I had put it down to
c) some people being new to driving a stickshift.*

I figure they put the 4000 mark on the tach where they did on account that's more or less where they wanted you to keep the revs if don't have the pedel down? I gotta admit the 996 is a dog at 30/kmh (19/mph) which is common for the *old towne* areas; doesn't feel right in either 1st or 2nd...

I've been driving a shift for a while now... about 40 years: my father's Pontiac Tempest (3 on the tree), my grand father's '58 pickup (3 on the tree), MG-A model (3 on the floor), Ford tractor (had a bunch of gears), etc...

NNH

Good thing I got my apology in early!

Macster

The ECU uses the pre-cat sensors to know it has control of the fuel mixture. It does this by varying the fuel injection pulse widths by a very small amount which results in the fuel mixture swinging from just a bit rich to just a bit lean.

How much is a bit? If the ideal air/fuel ratio is 14.72:1 the ECU will vary this by say 14.74:1 to 14.70:1 or even less -- I think it less but I don't have the exact numbers in front of me -- with the result the average is right on the ideal ratio which results in the converters getting an exhaust gas they can process sufficiently so the car and its emissions falls well within the allowable limits.

The ECU knows the converters are performing up to efficiency by monitoring the post-cat O2 sensors.

The ECU will (eventually) throw a fault code if it detects the O2 sensor is slow to respond, but this can take the ECU some time. The ECU is continuously monitoring O2 activity but the results are not examined and acted upon on a second by second basis. They are examined over a span of time.

If one was to query the ECU for its readiness monitor test results he will often find the results for all sensors/subsystems monitored (and especially O2 sensors) are not available (they haven't failed, the results just aren't in yet) because the driving cycle was not long enough, not close enough to the necessary behavior, to allow the ECU to finish the continuous monitor testing it, well, continuously performs.

The common assumption is the O2 sensor's either good or bad. If one is lucky, the O2 sensor fails outright and that's that.

But most of the time they fail over time. What happens is the O2 sensor starts out working ok. Then it misbehaves. If it misbehaves bad enough (and most don't -- at least at first, though their performance does degrade more and more over time) or if it misbehaves long enough (sometimes) the ECU will log a DTC.

In some cases, if the driving cycle (the trip) is short, or doesn't encompass the necessary variation, the ECU doesn't have enough "evidence" to flag the O2 sensor bad (emit an error code that could perhaps even point to the sensor) it may still emit an error. The error may be a bit of unintentional misdirection.

Remember, the auto maker's on the hook for converters in its emissions warranty. The O2 sensors are not covered. The manufacturer is going to err on the side of caution and potentially flag the converter before it flags the O2 sensor.

What the manufacturer wants to prevent is any damage to the converter that it may be obligated to pay for.

If the car is brought in and the converter is found to be ok and the O2 sensor bad, well, you pay for the sensor and the manufacturer is not out the cost of the converter.

The error code may implicate the converter when further diagnosis (perhaps even coming down to switching O2 sensors around to see if the symptom follows the sensor or stays with the converter) is done the culprit is found to be the O2 sensor.

The CEL will flash when the engine controller detects a condition that puts the converters at risk, or when the ECU determines it is unable to detect this condition, that the ECU is no longer in control and no longer able to monitor its control over the engine the engine. (Theoretically, the engine could continue to run fine the engine controller deriving the proper ignition advance and fuel injection timings from maps (hard-coded info) in the engine controller's non-volatile memory) but the engine controller is really concerned about ensuring the engine's within the emissions envelop and that nothing the auto maker is obligated to pay for if it fails is at risk.

In the case of a overly rich running engine -- say one fuel injector is leaking -- initially, the engine controller will try to lean the mixture out. It will lean (shorten the fuel injector pulse width) to all cylinders on the affected bank (determined by the O2 sensor inputs).

If it is able to bring the mixture's richness to a level the O2 sensors indicate is within the safety envelop of the converter, all is well. (Well, all is not well, really, because one cylinder on that affected bank is still probably running really rich, while the other 2 are probably really lean. There is a limit to how much the engine controller can go to adjusting the mixture from the ideal (determined when the engine was new) and if the engine controller exceeds this limit, it will light the check engine light and log one or more error codes. The long term fuel trims are where the engine controller stores this adjustment value. Typically, if the adjustment value exceeds +/- 10% the engine controller will turn on the CEL.)

The engine may misfire -- which will be detected by the engine controller and it can react in various ways. Misfire implies incomplete burning and excessive fuel making it to the converters. The engine controller will try some quick "fixes" but failing these to have the desired effect it will revert to limp mode in an effort to prevent damage to the converters.

I'm not advocating anyone continue to drive his car with the CEL on and not have any concern until or unless the engine begins misfiring.

I was just pointing out that had the converters been at risk of burning up, chances are very good he would have seen a flashing check engine light.

In the original poster's case, my money is on one (possibly more than one) O2 sensor being intermittently bad (though this still requires replacement) and not a converter being bad.

Sincerely,

Macster.

NNH

Thanks for your input Macster.
I'll report back as soon as I hear from the dealer.

1999Porsche911

This is a new code and indicates that the second CAT need replacing.

NNH

The dealer just called me to say that the other cat has indeed also died, so I now get another cat courtesy of Porsche and California's 7/70 emissions laws

NNH

Picked up the car yesterday morning. I've now put 150 miles on it, and so far (touch wood) it's behaving itself. Maybe the stern lecture I gave it, worked!

Macster

be sure, especially if one's on the hook to pay for any work that is done.

Cat failure is rare. Usually, if they don't get banged around and the engine remains in good tune they last and last.

You just know, though, the auto makers will strive to take some of that margin away...

Sincerely,

Macster

99firehawk

cats fail its not common but not uncommon.............cats fail way more often the o2 sensors on porsches

1999Porsche911

I would love see your evidenece to back that claim up.

99firehawk

few years of working on them, ive replaced probaly 4 o2 sensors and 50 cats