1999 Ford Ranger with low power and lean exhaust codes (P0171 and P0174) This was one of the most bizarre customer complaints I have had in my 20+ years as a mechanic. The complaint was low power but only with the headlights on! To be honest, I thought the guy was on something and I totally disregarded his complaint. I thought to myself "there is no way turning your headlights on can be associated with a low power complaint", I think I even told him that. But this one humbled me for sure and the take away is we need to pay more attention to our customers regardless of how strange of a complaint they may have.

Engine Performance Diagnostics chapter 16

symptoms 50

  • intermittent low power
  • low fuel pressure with the parking lights/headlights on
  • low fuel pressure with the turn signals on
  • check engine light (P0171, P0174 trouble codes)

fix 50

  • repair corroded main ground connection for the rear of the truck(fuel pump, tail lights, turn signals all share this ground)

test shown enh 50

  • how to test fuel pressure
  • how to test a fuel pump power and ground
  • how to perform a loaded circuit voltage drop test on a ground circuit
  • how to use fuel trim data to determine type of lean condition
  • what the BARO HZ data parameter tell us about the condition of the MAF sensor
  • how to find an open or high resistance in a circuit
  • how to read a ground circuit wiring diagram

Tools used

  • fuel pressure gauge (test kit)
  • scan tool
  • digital multimeter
  • heat shrink style wire connector
  • backprobing pins

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(Chapters 14, 15,16) Fuel Delivery and Fuel Pressure Testing

Related videos:

Fuel Pump Relay Problem:


MAF sensor trouble code (P0102) caused by a bad ground (GM 3100) :


Poor running, flashing CEL, no codes (bad computer ground) - Toyota :


How to test fuel pressure, injector pulse and spark with basic tools :


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For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available as an eBook or paper book.

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