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1999 nbs service engine soon light on

volfan

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volfan Enthusiast

volfan

Posted
Posted

ihave a 99z71 ext cab 4.8 with 68k miles that is bone stock and the ses light came on checked the owners manual and did all the normal stuff like the fuel cap and make sure it was tight.still didnt help so tried turning the ignition off and on like i had heard to reset still didnt help so i drove 3 miles to the autozone and hadem check the code and it showed po171 left bank 2.got back in the truck started home and light went off before ieven got amile away from the zone.so what exactly is this code?the guy that scanned it thought it might be a 02 or some kinda of sensor told me to go to the dealer.thanks for anyones input.

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted

Here ya go...

 

--------------

 

DTC P0171 Fuel Trim System Lean Bank 1

 

Circuit Description

 

The powertrain control module (PCM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. You can use a scan tool in order to monitor the short and long term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition. If the PCM detects an excessively Rich or Lean condition, the PCM sets a diagnostic trouble code (DTC). The long term fuel trim diagnostic parameter is an average of several of the long term speed load learn cells which the PCM selects based on the engine speed and the engine load.

 

Conditions for Running the DTC

DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0200, P0300, P0335, P0336, P0351, P0352, P0353, P0354, P0355, P0356, P0357, P0358, P0401, P0404, P0405, P0410, P0418, P0443, P0500, P0502, P0503, P1111, P1112, P1258, P1404, P1415, P1416 not set.

The engine coolant temperature is between 50°C (122°F) and 115°C (239°F).

The barometric pressure is greater than 74 kPa.

The mass air flow is between 5.0 g/s and 90 g/s.

The manifold absolute pressure is between 26 kPa and 90 kPa.

The intake air temperature is between -20°C (-4°F) and 90°C (194°F).

The engine speed is between 400 RPM and 3,000 RPM.

The TP sensor angle is less than 90%.

The vehicle speed is less than 137 km/h (85 mph).

The fuel level is greater than 10%.

Conditions for Setting the DTC

The average Long Term Fuel Trim cell values are above a predetermined threshold.

All the above conditions are present for 6 seconds.

 

Action Taken When the DTC Sets

 

The PCM illuminate the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions (load, RPM, temperature, etc.) as the previous ignition cycle that the test ran and failed.

 

The first time the diagnostic fails, the PCM records the operating conditions in Failure Records.

 

The second consecutive ignition cycle that this diagnostic fails [under the same conditions (load, RPM, temperature, etc.) as the previous ignition cycle that the test ran and failed], the PCM stores this information in Freeze Frame. The PCM copies any data previously stored in Freeze Frame and copies the data into the Failure Records. The PCM then overwrites the Freeze Frame. The only exception to this is if a Misfire DTC was already recorded in Freeze Frame. In this case, the Misfire data stays in Freeze Frame and the PCM updates the Fuel Trim data in the Failure Records.

 

Conditions for Clearing the MIL/DTC

The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail within the same conditions that the DTC last failed.

 

Important

If the last failure was during a non-typical driving condition, the MIL may remain ON longer than the three ignition cycles. Review the Freeze Frame/Failure Records for the last failure conditions.

 

 

A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.

A last test failed (Current DTC) clears when the diagnostic runs and does not fail.

Use a scan tool in order to clear the MIL/DTC.

 

Diagnostic Aids

This DTC sets if the engine runs out of fuel.

A fuel delivery malfunction causes this DTC to set. Thoroughly inspect all items that cause a lean condition.

For an intermittent condition, refer to Symptoms .

GMCTech Enthusiast

GMCTech

Posted
Posted

A bad sending unit almost never will cause this. Most likely caused by the need for a new fuel filter.

 

And................you buy gas at Wally World? I try not to buy ANYTHING from there that will touch any of my expensive possesions.

 

I go the Wal-Mart to get the best deal on cheap stuff. 'Nuff said.

 

Back to the situation at hand. You may need a fuel filter, and it is possible that you have a slightly clogged injector or fuel rail, but not enough to get a misfire code to set.

 

Hope this helps.

volfan Enthusiast

volfan

Posted
  • Author
Posted

thanks gm tech so i should be okay since the ses light went out?unless i notice gas mileage suffering?i last changed the fuel filter in the 50k mile range somewhere ill have to check my records to be exact.i also used some of the techron synthetic fuel treatment around 800 mile ago about the time i noticed the sending unit going.i now have 68k.thanks

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