So the volvo s40 is '02, 1.6 petrol, 109hp. Ever since I got it, gas mileage isn't very good I think. Recently got an error called P0420 according to my ELM327 scanner. It was about the cat, but it was appears it could also be related to a failed/failing O2 sensor. Now the Android app that I have used, Car Scanner, has some nice graphs of the data, but I have no idea if the ELM327 is reporting bad values or the sensors are damaged.

Here are screenshots of the data. https://imgur.com/a/BCWMOKC

It shows the Long Term Fuel Trim as constant +11.7 to +12.5% but usually its stuck at 11.7%. And no value inbetween. Whether the app is showing the data wrong, or the ELM327 is reading it wrong, I am not sure. I've also added a few screenshots of the O2 sensors. The weird thing about them is that when the car is not accelerating and is at a steady rpm, speed or even idling, the voltage is a pretty constant 1.28v for both. But when accelerating and the engine is in an open loop, they vary between 0.05-0.1v for O2 Sensor 1, and more variable data for O2 Sensor 2. But I've read that the voltage range is 0.1 to 0.9v. for typical O2 sensors(though there are narrowband and wideband). So what gives? Bad OBD2 scanner? Bad sensor?

The other issue I have is that when I lift my foot off the gas pedal completely while driving, and quickly press it(not floor it), it's as if the car cannot adjust the correct fuel mixture maybe, because I get some jerky movement of the car, almost as if someone is rapidly pressing and letting go of the brakes;like sloshing back and forth. This is very noticeable in 2nd gear. It also sort of happens but more mildly when shifting gears, I have to release the clutch very slowly(too slowly in fact) to avoid this jerky movement.

I have attached a csv of the logged data, should someone wish to take a look at it.

 

I had noted sluggish performance on my 1.9T S40 even though it did not set off any fuel trim or misfire CELs. I wound up putting in new spark plugs, new coils and replaced the wire harness to the coils. Power and fuel consumption went back to normal. Note the wire hardness connectors can crack over time and effectively lower the output of the coils. people have sealed with RTV, replaced connectors etc but I found it easier just to replace the entire harness from the firewall to the coils.

 

I will take that into consideration. The car is over 200k miles and the electrical cables in the engine compartment are not in the best condition for sure.

 

Could be a sign of a vacuum leak or MAF sensor issue.

 

I thought that as well. So I bought a vacuum tester. According to the tester, between 17inHG and 22 is the normal operation range of an engine.

At idle, my engine sits at 17 inHG, however it very slightly moves below, the meter has color coded ranges, and the one there says late ignition timing, the needle doesn't quite get to the middle, but it's not steady at 17inHG,and oscillates a bit to below 17.
I also used carb cleaner on the iac valve, as well as throttle plate, but that didn't seem to do much.