Babybenzc230

My 2002 c230 coupe was recently throw out code P0128. I realized my temp display is warming up too slow (took 30 mins to get up to mid 70 celsius). Since then I have have replaced the Mercedes coolant, brand new Thermostat, new spark plugs (torque to 17 ft.lb) and K&N air filter.

P0128 went away and the temp display is showing the engine warming up at the correct rate after all the fixing. But I have noticed two things a few days later. My mileage dropped from 32~33 mpg to 26~27 mpg. It is a ~20% dropped. Then later the CEL on with code P0420.

I have checked all the hoses for leakage but nothing found. Everything seems to be driven smoothly without hesitation. Any advice of what could cause the dropping in mileage and the code P0420?

sammydragon3

man i wish i had that code... hahaha my cat is f-ed up but no code

your cat converter is f-ed up

Glyn M Ruck

Unless the system is running very rich which would throw a fuel trim code it is time for a new Cat.

Babybenzc230

this is not what I wanna hear. I thought it was O2 sensor....Any recommendation of the cat? Is it a DIY project that I could tackle myself? I am a newbie on the car. Just learn to replace the thermostat, drain the coolant and replace the spark plugs from this MB forum.

sammydragon3

it could be your post o2 sensor but thats a slim chance (u need a good scanner to look at o2 readings)

well orginal cat is about a grand...aftermarket cats is a hit or miss

as for replacing it yourself its probally going to b a ***** to do

Babybenzc230

Sammy, post O2 sensor....Do you mean the downstream O2 sensor? The thing that puzzle me is the CEL and code P0420 only came on after I replaced the thermostat, spark plugs and K&N air filter. What did I do to mess up the mileage and the cat?

sammydragon3

yes downstream

as for replacing those things i wouldnt think it would affect the cat
my cat is f-ed up and still not throwing a code

i'd just clear the code and c if it comes back...only thing u can do

wippet

P0128 = Cooling Sys Error / Thermostat Perf
P0420 = 1/1 Catalytic Conv Eff
Try to replace the original MB air filter

Babybenzc230

"wippet: Try to replace the original MB air filter"

I have just replaced the orig. MB air filter with K&N air filter.

Glyn M Ruck

If it was an upstream O2 sensor or temp sensor causing rich running you would get a fuel trim code. It could be a downstream O2 sensor but that would give you a O2 sensor code & not the Cat code.

The only way the K&N filter could cause rich running is if it was hopelessly over oiled.

Does your car warm up quickly with the new thermostat & run at a temperature between 87 & 90 deg. Celcius?

Babybenzc230

Glyn, my car warm up to 85 C within 5~10 mins of driving after the new thermostat, compared to ~30 mins to 75 C on the old one. Maybe the new thermostat somehow does not close/open properly?

I install the K&N filter right out the box, unless the manufacturer over oiled it (which I doubt)? Will my work on replacing the spark plugs causing it to run rich?

So from what you are saying it's not the o2 sensor since there are no code on that, but it's the faulty cat

Glyn M Ruck

It's almost certainly a faulty Cat. You can get your exhaust shop to weld in an aftermarket Cat if you don't want to spend a fortune on an OE one.

Babybenzc230

Thanks Glyn, I will give it a try. An aftermarket Magnaflow will cost me ~$250. As a first time benz owner, I am getting a bit frustrated on this code/problem chasing. I hope this won't turn into a money pit.

Glyn M Ruck

Plain & simple - If you get over 80K miles out of a Cat you are on borrowed time.

Good luck!

samaritrey

So 167k is on borrowed time x2 oh crap

lol

Glyn M Ruck

Trey - for some reason the multiple Cats on the 112 seem to do well but yes you have done remarkably well. Friend's 112 has just thrown it's Cat warning at just over 200K Km.

Anthony Ochs

I spent $12k in repairs, just in the first 2 months. This is the biggest pos I have ever purchased!!!!!