This error code cites problem with "Short Term Fuel Trim". Procedure in Volvo "Green" book cites use of Volvo diagnostic scanner (which I don't have of course) to read voltage on MAF sensor I believe (2.3 -2.4 V @ idle). After that it says to measure output pressure of high pressure fuel pump. After that it points to DC diagnostics of MAF sensor & wiring to sensor . . .

I have noted that the high pressure fuel pump under the driver's seat has been making a high pitched squealing sort of sound intermittantly. It was making this noise today when I checked the car after my wife told me that the "Check Engine" light came on this morning.

I noted that the car is idling sort of rough.

The pinout diagram of the MAF sensor in the Volvo manual is not very illuminating as it does not show any circuit of the MAF sensor, though it's got to be pretty simple. I have an identical '94 960, so tonight I will compare the DC characteristics of the MAF sensors.

I also checked to see if there was fuel on the vacuum side of the Fuel Pressure Regulator and it was dry.

I'm inclined to think that the high pressure fuel pump may be the problem since this squealing has been going on intermittantly over a period of several months.

Any others out there have any experience/insights on this?