Yamae

It is operating hours + non operating hours. My suggestion is for the preventive maintenance and this is rather a safe side. The heat degrades the capacitor and you also should pay some attentions on this. On the other hand when you think about the ratio, the non operating hours are much longer.

Your car 99 has only 72,000 miles. Suppose your average speed was 36 miles/H, you have been driving only 2000 hours. If the average speed 20 miles/H, then 3600 hours. 99 means 13 years old and in hours, it is about 114,000 hours. The ratio of the driving hours is only a few percents. Suppose you car has 150,000 miles still the ratio is 5% or so. What I mean to say is that the running hours are not the major part of the total hours but partially it affects. At least it is affected in accordance with the "Arrhenius Equation".

I am still studying and I still do not get the conclusion yet, but there is another factor that Celsior 98-00 has a less risk of the ECU to fail as far as I investigate the inside. If your 99 LS400 has a similar design of the ECU, there would be a less risk too due to the safer design of the low voltage regulator circuits. The series regulators are used and they don't generate the ripples as well as they absorb ripples from the power source. I still don't have enough bad samples of ECUs for 98-00 and further study is needed more.

Regarding the judgment that you replace them or not, it is all up to you. Due to the facts that I wrote above, you don't need to replace them very quickly. But they say, "Prevention is better than cure or Look before you leap". Since you are a guy who prefers to do a preventive maintenance job, it would be better to replace them before they fail and you don't need to worry about the future problem. The replacement is easy for those who have experienced PC mother board caps.

The attached photo is part of 98 JDM's ECU where those “quaternaty ammonium salt / compounds" capacitors are mounted. I think yours is mostly the same. I have one more suggestion that you'd better to check within this area. The resistor indicated by an yellow arrow at upper right corner tends to increase the resistance after the long term use changing the color from light blue to yellowish or pinkish. You'd better to replace it too if the resistance is bigger than normal. Probably this is more important than capacitors in case of 98-00 according to my investigations so far. You can see more on this at my blog page writing for my good old American DIY friends.
http://blogs.yahoo.co.jp/mae_yas/3189638.html

python

iTrader: (4)

i know several 1st gen Ls' and Sc's that have well over 300,000 miles that have never had the caps changed. the fact that lexgx had his car worked on..didnt have the issue before hand,and now has this issue..usually means whoever worked on this thing screwed it up.
i would jump off a ledge here and say the wires are crossed and/or the coil(s) have failed or the connectors are broke causing an issue. take the car to an exhaust shop to make sure the cats are working..u can check em by reving the car and making sure u have adequate exhaust coming out of the tail pipes. the ecu is a last resort type of diag..not the first place to start diagnosing a car. elimination of everything else..then maybe the ecu is to blame

xXTACOXx

So was this problem ever solved.
Same scenario for me except I did the Tune up. I triple check my work. Hate too dig back into it to solve nothing.
Should I just replace the cats ? I just don't want too have them fail shortly down the road.
I've only driven maybe forty miles while they have been clogged.
Been driving the blazer.