I recently fixed my upstream lambda sensor which was reporting a P1647 fault (described in a separate thread), cleared the fault code, and was happily diriving for the first time without an engine warning light, when a few miles later up popped the message "Performance restricted", and another engine warning light, with a DTC of P0174 (System too lean, bank 2)

Researching this I know there are many possible causes, but the most obvious to me is that the lambda sensor that I replaced the Denso one with is not giving the right signal. The one I bought was from ebay here. When it turned up it looked exactly like the Denso one I had taken off, and had a heater resistance of 2.5 Ohms which seemed about right. but did not have Denso written on it like the old broken one.

I was thinking of buying another sensor from Eurocarparts, this time a Denso one, but I don't want to spend another £70 for no reason. What do you all think?

After replacing sensor, did you do a hard reset?
Attach an OBD reader and set it to log LTFT and STFT on both sides while you drive; might give a clue to what is happening.

If you are getting an additional code you either have another problem or the wrong sensor

Thanks for that. How do you do a hard reset?


....it's alright - I looked it up!

Thanks for that. I'm suspicious of the sensor so I think I might buy a new one - a genuine Denso one this time - and if that fixes the problem try to get my money back on the other one.

Used to get the 174 on my RR SC. A bottle of Lucas, or Gumout's Best always seemed to clear it within a day or two (need to reset of course). Lately, found SeaFoam also cleans the injectors better than the other two. Might try that first. Dirty injectors cause a lean read . . .