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.
Update on it: spent some time testing various electronics and found a faulty O2 1 sensor (the one I already replaced). Replaced the O2 sensor with a genuine one and looked at the codes, still seeing the O2 sensor code yet when graphing the engines O2 sensor voltage via an OBD scanner, everything in the idle is completely normal and I see no issues with the readings to indicate a problem with the engine.
In response btw to the questioning about the car it’s a ford street ka 2003 with a 1.6 litre duratec engine. Also there is pretty much no community with this car anymore as most of the street ka’s have died out and few people even know if their existence in any country.
The list of parts that have been replaced now are as followed: O2 sensor, PCV valve, thermostat housing and sensor, injectors, fuel pump, and some cabling for the injectors too.
Checks that have been done include: intake leak test, exhaust leak test, O2 readings test, MAP sensor test, intake has been cleaned, and fuel pressure is as expected.
I’m checking fuel trim as I type now
Yup so normally sensor 1 and 2 bouncing between 0.2 volts and 0.9 volts idle… But… Long term fuel trim at 18%
That indicates a problem remains. To keep the O2 sensor feedback loop happy the engine computer must inject 18% more fuel than it thinks should be required. The amount of fuel required is a calculations based (mostly) on the MAP, throttle position, and engine coolant sensor readings. The computer assumes a known amount of gas is injected (known by the engine design engineers) for every 1 msec fuel injector is pulsed, so if that is incorrect on your car (faulty fuel injectors or incorrect fuel pressure) that would show up as a fuel trim problem also.
I presume sensor 1 is pre-cat. That’s the one that is used in the afore-mentioned o2 sensor feedback loop. On my Corolla the pre-cat (the only one on my car) its o2 sensor at warm idle cycles between 0.05 volts and 1.0 volts. The O2 sensor isn’t the cause of the cycling btw, it is the engine computer’s doing. The engine computer cycles between injecting a little more fuel than necessary to a little less, and that shows up as the O2 sensor voltage cycling. 1.0 volt are periods of time when there’s some oxygen in the exhaust stream, and 0.05 volt are times when there is no oxygen in the exhaust stream. The fact that it never goes below 0.2 volts on your car suggest there is probably a path for a small amount of air to get into the exhaust stream. That would be consistent with your 18% fuel trim. Common ways for air to get into the exhaust stream
Some ideas on what you might want to try next
I expect you know this could all be done much faster by hiring a experienced shop. But you may feel it is more fun to do it on your own. If so, remember that working on car can be hazardous, so take the appropriate safety precautions. I think your car has future classic appeal, so you got that upside going for you.
Add’l note: There’s a slight possibility you’d already solved the problem by replacing the O2 sensor, and it just is taking some time for the engine computer to adjust to the new sensor.
The person in this video is using the torch to remove oxygen from the tip of the O2 sensor.
Tester
Now, that’s an interesting one to be honest. Exhaust leaks I’ve already checked for, found none. As for the intake I pressurised it with my compressor and blocked the throttle body with a make shift seal. The intake manifold pressurised and the pressure did not go down so the PCV and intake system clearly isn’t having any issues.
After checking over the engine I spent time just watching my monitor and eating lunch while listening to it idle and the short term fuel trim doesn’t move from -2 to 3 % so I’m hoping that the long term fuel trim will stabilise based off that knowledge and the engine will be fine. If not now that you mention it I have all the kit for tuning that engine so I may as well re do it’s tuning.
Also it’s already been to two professional shops, one of which being ford themselves so I quit letting other people try fix it