Hey Guys

234K on my M103. It runs like a raped ape, idles smoothly. Not a single drivability issue at this point.

Just recently I've done: Spark Plugs, Head Gasket, Dist Cap + Rotor, Air Filter

A few years ago the last time it failed smog: The O2 sensor was replaced with a Mustang Unit, EHA, and Engine ECU.

I'm kind of leaning toward O2 first. It has been about 70K since it was replaced. Or maybe CTS?

Original: Catalytic Converter...could a cat alone cause to fail BOTH HC & CO?

Idle:
HC Read: 392 Limit 120
CO Read: 6.09 Limit 1.0
CO2: 10.8%
O2: 0.2

2600RPM
HC Read: 185 Limit 140
CO Read: 3.14 Limit 1.0
CO2: 12.8%
O2: 0.1

70k is a lot for an O2 in my opinion. I would change that and also look at your plugs to make sure they aren't fouled up.

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1990 190E 3.0L

What is the factory mileage interval for replacing an o2?

Plus were done at 216k and the ones I took out were nice and clean. Top end was all redone.

I think it's 30k on the Bosch packaging, not sure

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1990 190E 3.0L

Well O2 is a logical place to start anyways. I ordered a new Bosch unit off eBay for $21 shipped. Yes, I'm cheap.

I'll try to find the coolant temp sensor test too. Anyone else?

Bosch recommends the following:
unheated O2 sensors 1976- early 90s every 30k-50k miles
Heated O2 sensors (first generation) mid 80s- mid 90s every 60k
Heated O2 sensors (second generation) mid 90s and up every 100k
This assumes that the car is in good running condition.
So worse case scenario you just buy a new catalytic converter. I was able to get on installed for about $220. Passed smog with no problems after that, but the nightmare I had before getting that new cat was unbelievable. I guess the m103s may be known for failing smog as mine failed 3 times consecutively and every year that I had to get it smogged it failed. So my advice is that if you still fail....just bite the bullet and get a new cat if everything else is in good condition otherwise you may be in for a battle. Also, make sure you are running the right plugs, non-resistor type and the brand specified, although I have not haad issues running the resistors, but there seems to be a divide about using those amongst many forum members. Good luck.

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1990 190E 3.0L