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791 Posts
Discussion Starter #1
1999 LS Auto with under 150k miles.
It seems that the (new NTK) upstream O2 sensor is not seen to be responsive enough as per the Scanner.
Could this be ECM? ECM is not able to interpret the actual O2 sensor reading?

– When the car is idling, the STFT slowly creeps up and the car earns the P0171 CEL at idle
When driving steadily, CEL goes off.
The car has no drivevability issue.
It also has set of new reman Fuel injectors.
Fuel pressure and regulator were tested negative.

Am I correct that P0171 means air is more than needed?

The shop could not nail it down.

How common is that the people have changed the ECM?
Anyone repair your ECM?

4,189 Posts
Have you tried searching the site for this code where others had the same code?

Also - This is taken right from the factory service manual with a quick internet search

Reasons for code:
  • Fuel Pump insufficient flow/pressure
  • Fuel Feed Line clogged, and/or leaking
  • Fuel Pressure regulator stuck open
  • Fuel Filter clogged
  • Fuel injector clogged
  • Gasoline doesn't meet owner's manual spec
  • Primary HO2S (Sensor 1) deteriorated
  • Incorrect Valve Clearance
  • Exhaust Leak


791 Posts
Discussion Starter #9
Does the flow-chart specify? You're reading too far into it. Just follow the chart, it's a step-by-step guide to diagnose.
One thing intrigued me is that when I am driving the car, Fuel Trims reverses to the extent CEL/P0171 goes off.
Does this mean i should look elsewhere?

LTFT drops to 14 and CEL goes off.
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