So started getting a P0171 fault on my 2006 2.4 Alphard in August 2025. To try and solve it;

1) Replaced EVAP charcoal canister (it was knackered and leaking so replaced it)

2) Replaced PCV valve (fuel consumption got way better after this, went from about 280 miles per tank to about 320)

This was in August 25. Engine Light was still coming on after driving for a while (30 minutes or so) and the Ancel JP700 scanner showing P0171, but fuel consumption was so much better and had other things to do so left it. In the last 6 weeks though I picked this up again:

3) Replaced MAF sensor

4) Ran cat cleaner through the system

Same symptoms. So I then

5) Replaced upstream O2 sensor

This is where it changed. I reset the ECU, and then drove around for 45 minutes while it relearned. It went through the stage of sorting out the idle (was too low for a few minutes before sorting itself out) and it ran "lumpy" for a short while, but by the end of 45 minutes or so it was running excellently, and no engine light and no fault code on the scanner. Excellent I thought! However.....next day, I started her up and although it ran fine, the light came on after about a minute, and sure enough, P0171 is back. Since then, (a couple of weeks ago), every time we drive it, same thing happens, and the fuel consumption is now terrible - we're lucky if we're getting 180 miles per tank!

Clearly the ECU is injecting far too much fuel because it believes the engine is too lean, and since replacing the O2 sensor this has got worse. My guess is that the previous sensor was faulty and wasn't reporting as much O2 in the mix as the new one is? Or because I'd reset the ECU, it's now learned differently and wants to stick more fuel in.

Air filter is fine, was new at July 25 looks clean. Was last serviced in July 25, oil changed. What's next? It was checked for vacuum leaks when the EVAP canister was replaced, but I guess I should get it checked again (and for exhaust leaks) in case that's causing an issue. Maybe the new sensor is bad? But that doesn't make a lot of sense as the old one was present also for this fault code. Is it time to move onto fuel pump and filter? What else could be going on? Google says outside of all this then could be an knackered cat, or ECU itself damaged, or possibly injectors.....anyone got any ideas on what's most likely?

So you changed the O2 sensor on the front exhaust manifold - that is likely to be the bank 1 sensor?

There is another underneath, which should be referred to as 'bank 2' (I think) so that may be a similar replacement. I only changed the front one and it passed the MoT with no issues.

Check:

the cleaned MAF sensor is mounted tightly and electrically connected Ok.

the replaced sensor is screwed in fully and no air is getting in that way - I had to ream out a new thread on mine as the previous person had jammed and cross-threaded it.

that the air filter is also not loose and fitted back Ok.

the electrical connections are good - remove and re-seat them

You are right that the system thinks that there isn't enough petrol going in (in relation to air), so it's a matter of working out why that would be.

I hope that helps.

Thank you! Yes I replaced the top one (bank one). I could try and replace the one you've shown I guess, but apparently that would be more of a P0174 code, and I've read that it only monitors the post-cat exhaust gases and doesn't affect the fuel mix. Might give it a go anyway. Thanks for the tips, I will check the O2 sensor I replaced is fully seated.

Hopefully you won't need to use this but if you need to re-cut the thread on the lambda sensor hole or the sensor itself they are an M18 x 1.5 tap and die.

I got mine from Amazon - https://www.amazon.co.uk/dp/B0C9T6VBGR?ref_=pe_112168041_1111392281_fed_asin_title&th=1

OK so 2 days ago I checked the sensor - removed it and put it in again, it was fine, not cross threaded or anything like that. I also gave the connector on the car side a good spray with electrical contact cleaner. Since then the fuel consumption seems to be better, but still getting the engine light. This morning I checked the codes with the scanner and now have got a P0031 error as well as a P0171.

Going down a google rabbit hole, I got the multimeter out and checked the resistance across the black wires on the sensor side of the connector, which reads 3.2 Ohms. The old sensor I took out reads 2.7 Ohms.

I can't seem to find a definitive spec for what the sensor reads, but again google is saying that the Alphard's ECU (2AZ-FE engine) is expecting the resistance to be 0.8 and 1.4 Ohms at 20 degrees Celsius.

The sensor I put in was a cheap aftermarket one (branded "BIVVI") form Amazon, I was too tight to buy the real thing from Amayama! So am wondering if me cheaping out might be the problem here. Does anyone know for sure what the spec on the sensor needs to be? Suspect that the one I replaced was bad, and the new one is just not manufactured to the correct tolerance needed by the Alphard.

OK more digging around produced this:

https://www.autoequipment.com.au/image/data/Support/Toyota%20Planar%20Sensors%20-%20Part%203.pdf

Which appears to be the specs for the upstream O2 sensor used in these engines (more correctly called an air/fuel ratio sensor I have learned). It seems there are two versions, a "cup type" and a Planar sensor (the cup type being an older design). The resistance specs for the cup type are 0.8 to 1.4, but for the planar type it's 1.8 to 3.4 ohms. I've confirmed that my alphard uses the the Planar type, and also I re-measured as the engine was a little warm earlier and the new sensor I put in measures 2.8 Ohms so is within spec. So scratch that theory!

Also checked the voltages on the wiring loom connector and there's 12 volts getting to the heating part of the sensor, and I'm getting 3.3 volts on one sensor pin and 2.9 on the other, and 12 volts on one of the heater pins and ground on the other, which appears to be correct.

At this point I'm back to either some sort of vacuum leak, or something broken that's unconnected to the sensor. Unfortunately my scanner (an Ancel JP700) only lets me read and erase fault codes on the Alphard, it's datastream OBD2 function doesn't work, so I can't see what the sensors are reporting to the ECU when the engine is running.

I'm probably going to have to quote Clint Eastwood at this point and declare "a man's gotta know his limitations" and take it somewhere that can do a proper vacuum leak test and get the full diagnostic data from the ECU.

Yep, it sounds as though you have reached the limit of what can be reasonably done at home.