Hi, head scratcher here, (for me anyhow),vehicle arrived eml lit, po131 present, in live data bank one oxy sensor reading constant 400mv, adding fuel or creating air leaks did not alter this reading. Assumed oxy sensor failed so renewed with NTK unit, this new unit refused to operate at all flat lining at ov regardless of WOT or idle, or RPOT (rapid pumping of throttle!).
Aha, thought I as I discovered a split pipe to pcv valve, renewed this item, no better!
Also plugs in very bad condition, yet eml came on again after drive of about 10 miles, yet when monitored the oxy sensor is now behaving well between 130mv to 800mv.
Sorry about long post, will not object to long replies!
Thanks phil

hi

The 400 mv could have been a default ECU value when current fault logged for o2 sensor.

I would check fuel trims next for any clues, freeze frame and current values.

Cheers Aidan

spoken again to customer, she reports huge fuel use but no significant loss of power, vehicle has not been serviced for some time so am now going to change oil and fuel filter (oil was topped up prior to long journey this week), thinking may be tappet jack due to wrong or poor quality oil.
Thanks for post Aiden, will recheck these.
Phil

(tappet jack) On solid lifters