A P0131 could be an O2 failure, you replaced so that’s not it…It is a wiring problem. High resistance or short to ground.
You said you checked the wiring… How? Just visual? That is not enough. Probe the lines at the ECU to the O2 sensor plug. Check for resistance and ground from all 3 wires. I’d bet you find a problem. Fixing it may be difficult but it is cheaper than throwing parts at it hoping that fixes it.
The Ford Ka is a tiny car that was never sold in The US. The StreetKa version is a tiny convertible, but to make this whole thing more confusing, depending on the year when this mystery Ka was manufactured, it just might be a re-skinned Fiat, rather than a Ford design.
Pug1 is asking about vehicles that hardly anyone in the US is going to know much about. Granted engine problems can be common to almost any gas type . But the real help would be from a Forum dedicated to the vehicles in question.
Doesn’t Europe have forums like this ?
Typo. You are correct above, " fuel trim". That’s probably the most helpful first-test.
The problem with trying to diagnose air/fuel mixture symptoms is that while there are maybe half dozen common causes, there are probably 50 others which are not so common. You just have to start with the most likely and test them one by one, moving down the list toward the least likely. I presume the Ford dealership eliminated some of the possible causes, but are not able to continue their diagnosis b/c their labor fee would be more than your budget allows.
To avoid wasting your time, It’s a good idea to make sure everything that you know needs doing is done before starting your debugging effort. For example if you know the spark plugs & timing belt need replacing, do that first. Same goes for checking the valve clearances. Also a good idea to make sure the engine computer knows the correct coolant temperature, b/c that parameter is an important input to the mixture calculations.
How did you test the O2 sensor swing s in voltage? A DVM isn’t the best method for that, and could explain why you missed the voltage peaks in both direction. I use an o’scope for that. Many OBD II scan tools have that function as well.
For best help here suggest to also post the car’s model year and transmission configuration. We don’t have the “Ka” in the USA as far as I know. I think of a “Ka” as a sort of minimal-configuration Ford Escort or Focus. Do either comparisons apply to your particular “Ka”? The reason I ask is b/c I might be able to find the service data for its cousin-car. Good for you for wanting to keep it on the road & best of luck!
Your wikipedia article link above says the fiat version of the “ka” (second generation) doesn’t have a 1.6 engine option. The only 1.6 engine option seems to be with the first generation (1996 -2007), a Zetec 1.6 engine.
The only things that are left to check with a P0131 AND a lean code that you haven’t already mentioned are:
This assumes the list of repairs/checks in your original post were done properly and are still functioning - intake/exhaust leaks, PCV, injectors, etc.
I suggest getting the oxygen sensor working properly first thing. Either your cheap O2 sensor is defective, or there is a wiring problem. A scantool should tell you the Voltage coming off of the O2 sensor. It should be the same as what you measure with a digital Volt meter.
Regarding the sudden increase in fuel use, I would check that the fuel pressure regulator isn’t failed and leaking fuel in to the engine vacuum line. If your O2 sensor was working, this would give you a mixture too rich code.