P0171 lean condition / sluggish MAF sensor

[HKB]

Since the summer I've been trying to diagnose and fix the cause of a P0171 error on my V70 P2 petrol engine (140HP B5244 S2). I started with all of the obvious checks:

Inspected and smoke tested air intake for leaks (also tried using butane gas to detect leaks) - no sign of any
Replaced air filter and fuel filter
Ran injector cleaner through tank
Tested fuel pressure
Replaced cracked fuel filler cap seal
Cleaned throttle body
"Glove" tested crank case vacuum - good vacuum but not too strong and no whilstling sound
Checked MAF sensor output values using OBDII scanner - seemed OK
Attempted to clean MAF sensor with electrical cleaner
Inspected spark plugs
Checked operation of evap purge valve solenoid and inspected the hoses

I then focused on the front O2 sensor which appeared "tired" when I looked at the graph using the OBDII interface. After replacing it with a new Denso sensor from Walker Products and resetting the ECM, it seemed much more responsive, but the error came back after 300 miles with a clearly negative LTFT and oddly reduced fuel consumption (38 mpg on motorway).

I finally turned my attention back to the MAF sensor last week, thinking that if it were under-reading the air flow, the ECM might reduce fuel injection, creating a lean condition which would be picked up by the O2 sensor.

The results of graphing MAF sensor output against engine RPM reveal that it was clearly dying. The old sensor (see first attachment) responds very sluggishly, is almost certainly under-reading and clearly filters out a lot of the changes in air flow. The new one (a Denso, Toyota branded - see second attachment) responds faster than the change in RPM. When I looked closely at the old sensor wire and bead I could clearly see that it was dull on the intake side and shiny on the opposite side. It was probably 20 years old and cleaning it had little effect. Fuel consumption is back to 34 mpg (even in heavy stop start traffic), which is better than my long-term average of about 32 mpg.

I don't want to conclude that all is now well, but I've done nearly 500 miles over the weekend with no error code.

What I have learnt from this is that the static MAF sensor value at steady state engine speed can be misleading and doesn't give a complete or clear picture of its state of health. The dynamic response is much more revealing. Probably graphing it against throttle position would have made more sense.