1999 Ford Explorer Check engine light comes on when cold ou

Tiny
DIXONAB
  • MEMBER
  • 1999 FORD EXPLORER
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 117,000 MILES
The check engine light comes on when the weather dips below -7 degrees celcius (or so). Usually when I hook it up to a diagnositc machine, I get one of the following codes: P0171 or P0174.

Today while driving to work, the light came on again. I hooked up the scan tool, and it came back with NO codes. The light was on, but no codes were present? How is that possible?

My O2 sensors seems to be working fine according to the scan tool (the graphs are not just flat, there is some movement), so what else could be causing the problme? Specifically, what could be causing those codes? Why does this only happen in the cold weather?

Help? Thanks.
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Thursday, December 17th, 2009 AT 8:42 AM

1 Reply

Tiny
JACOBANDNICKOLAS
  • EXPERT
PO171 - Fuel Trim System Lean (Bank 1)

PO174 - Fuel Trim too Lean (Bank 2)

Based on your description, my first guess would be the temp sending unit. It tells the computer ambient temp and coolant temp. From that, the computer determines how rich or lean to make the fuel mixture.

If that isn't the problem, check for an intake leak or other vacuum leaks.

Let me know what you find.

Joe
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Saturday, December 19th, 2009 AT 1:03 AM

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