I've searched this site pretty extensively and haven't solved this yet, so I ask you about this volvo i've had for about 6 months now.
I knew when I purchased it that the two codes being thrown were p0128-Engine coolant temperature, and p0420- Cat efficiency Bank 1.
Inspecting mechanic said it needed a new cat to clear the p0420....
I went ahead and replaced the thermostat and changed the coolant to "fix" the p0128, and cleared the codes, I think the jiffy lube state inspector did not notice the that the catalytic converter test was Incomplete when he passed it. Lucky me I guess.
But now I know that i'll probably have to fix before it gets inspected again. Eventually the p0128 came back on, so I went and got a new ECT sensor. Still doesn't clear the p0128. What am i missing? Coolant temperature sits at a 3'oclock position pretty regularly and quickly.
I have no idea where to start on the p0420 besides understanding its the Pre-cat o2 sensor that is throwing the code, is there anyway that these are related?
Thanks for your help! - See more at: P0128 and P0420 on 2001 Volvo S40 - Car Talk

 

You might have either a bad thermostat or wrong temp t-stat right out of the box. A new 195* fixed up my code P0128.

The easiest way to "fix" the P0420 is with a Oxygen Sensor Spacer (CEL Boss)

It goes in between the bottom O2 sensor and the cat convertor. This is the O2 sensor after the cat, not the top O2 sensor before the cat.

 

Thanks Hudini, I've seen that CEL Boss before, but wondered if it would be masking some other running problem, lean/rich mixture, etc?
Also, by Temp t-stat you mean the engine coolant temp sensor?
Edit: reading your post again and now i'm thinking you meant the thermostat I put in might have been bad out of the box, or set to open at wrong temp.. Is that what you meant?

 

Also, what do you mean a new "195*"?

 

The bottom O2 sensor triggers the "cat below efficiency" trouble code. The problem could be the actual sensor. It could be the cat. Personally, I do not care as long as the car passes testing. Your second issue may also contribute to this problem as the engine needs to be at proper operating temp for proper combustion.

195* equals 195 degrees Fahrenheit. T-stat equals thermostat. Some brands have the opening temp stamped on the t-stat or printed on the box. You do not want the 180 degree one.

Sometimes thermostats can be bad from the factory. It's happened and causes all kinds of headaches as you KNOW it's new so that cannot be the problem.... I know that my P0128 code was fixed with a new thermostat.