Limp Mode

[SHOgoFast]

I am interested in some specific details as to what happens durring Limp mode and if all CEL situations cause it. All that I could find on the old forums is that it pulls timing and runs rich. There has to be a more sophisticated answer.

The reason I ask is that I was previously throwing lean codes and slow O2 responce codes. I discovered my BOV was vac leaking badly and fixed that issue and for days had no CEL. Prior to finding that vac leak the the CEL would return after no more than two drives. Now, yesterday under regular driving conditions the CEL returned (lean both banks and slow responce O2 on one of them). With the CEL on the car feels sluggish and my wideband seems to swing rich and lean more wildly. And to clarify it goes to the extremes of the guage where it did not do so before under regular conditions.

If I do give it a decent amount of throttle, really just enough to build positive boost it smells rich. Goes quite lean at shifts, but I think that is normal. Could be wrong.

[Tony2005]

With a slow response O2 code, the O2 sensor is probably on the way out. You should test it to confirm.
http://www.autozone.com/servlet/UiBr...3d801bca83.jsp

Here is a good article on most O2 sensor codes from v8sho forums.
http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm

And from the 1996 Ford CD.

"Failure Mode Effects Management

Failure Mode Effects Management (FMEM) is an alternate system strategy in the PCM designed to maintain vehicle operation if one or more sensor inputs fail.

When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value and continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the normal engine running strategy.

All FMEM sensors display a sequence error message on the scan tool. The message may or may not be followed by a Key On Engine Off or Continuous Memory DTCs when attempting Key On Engine Running Self-Test Mode."