P0116, P432, P422 Code Solution

So I've been getting cat underfunction codes on my 99 355 for some time now, and they appeared to be most common when cruising at part throttle. Then I got a code for below temperature range, and I looked at what others did. Most dove into the car to replace the temperature sender(s), but I noticed that my temp gauge really was registering low at cruise and part throttle. So rather than attack the symptom, I decided to attack the possible cause. I looked high and low for an 82 degree C thermostat, not wanting to pay the Ferrari tax. Finally, thanks to the efforts of others, I found a Gates TH13182G1 at autospares.eu for a total, with shipping of 37.57 from ebay. Came from Riga, Latvia, but was made in Italy. Better? So I rigged a system to drain the coolant tank and dump into a gallon container through suction, and removed the plastic cover around the tank and took the hoses off the tank and set it aside. Unbolted the power steering reservoir and pulled it down to gain access to the three thermostat bolts. Did not have to drain it. It appears there was no gasket there before, and the box from Riga had a gasket that didn't fit. The rubber surrounds do, the flat one being my choice, and I tossed it in. Apart from losing MORE nuts down the Valley of Death (in between the cylinder banks,) Reversed the process and started it up. The car had a correct 82 degree thermostat, but on the side of the wax cylinder you could see a scrape mark suggesting it was binding. In heated water, it opened a little late, and stayed open longer than it should have. Binding indeed. So now the car stays between the 11 o'clock mark on the temp gauge and 12 straight up, 90 degrees, and all the cat codes have gone away. The engine was running cool, therefore rich, therefore the cats were not lighting up and I was wasting gas. Hope this helps. Many thanks to those who researched compatible thermostats among the various vendors. I've seen units that failed to open, but never one that was slow closing.

 

I was worried about this too. I compared the height of what I took out and the Gates, and they appeared to be similar. It works just fine, to my experience. I wouldn't mind a thermostat that warmed the engine up faster.