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Mike S.
Mike S., Chevy Mechanic
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1996 olds achieva: check engine light..spark plugs..cam

Resolved Question:

I have a 1996 olds achieva with a 2.4 liter engine the check engine light is on with a p0171 code. The motor starts hard once it's running it sometimes has a small mis but as the engine warms up it seems to go away. I have replaced the fuel pump the fuel injectors and the spark plugs the coil pack the inginition control module and the cam positioning sensor. But still if you try to accelerate to fast the motor either stalls or starts to misfire really bad. Until the motor gets above 1500 R.P.M. then it runs fine.
Submitted: 4 years ago.
Category: Chevy
Expert:  Mike S. replied 4 years ago.
P0171 System too Lean (Bank 1)

Information for Code p0171

Code p0171 means that the computer has noticed (by reading the oxygen sensors) that there is too much air going into bank one (the bank that includes cylinder number one in the firing order in v style engines). This information is also applicable to code p0174 as this code is the same except of the opposite bank of cylinders.

 

Engine control computers need to have a specific ratio of air to fuel in order to work properly, and if there is too much of one or the other then it will set a code.

There are several very common things that cause this code:

Dirty Mass Airflow Sensor
Vacuum Leak
Bad Oxygen Sensor(s)
Running Out of Gas

Some other things that can cause this code:

Clogged Fuel Injectors
Leaking Exhaust System
Evaporative Emissions System Malfunction
EGR System Malfuncion

The first thing that I would do is make sure no one has ran out of gas in the vehicle recently. This will cause this code almost every time. The next thing is to clean the MAF sensor. You can buy cleaner specifically for this procedure.

Then you should check for vacuum leaks, especially ones that are after the MAF like tears in the tube that goes from the airbox to the engine. If this is torn it will let air in that the computer cannot tell is coming into the engine.

The next most common cause is bad Oxygen sensors. If they are not working properly then they will not be reading the amount of oxygen in the exhaust gas.

These are the easier things to check. If none of this fixes it then it would be a good idea to take your car to a professional.

Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 5058
Experience: ASE Certified Master Technician
Mike S. and 4 other Chevy Specialists are ready to help you
Expert:  Mike S. replied 4 years ago.

Mass Air Flow (MAF) Sensor

TESTING



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Terminals C and B supply voltage to the MAF sensor

 

  1. Visually check the connector, making sure it is properly connected, and the terminals are straight, tight, and free of corrosion.
  2. With the engine running, lightly tap on the MAF sensor and wiggle the wires at the connector; if the idle speed fluctuates, it is possible that the connector is damaged.
  3. Backprobe using a DVOM set to the Hertz scale between terminals A and B. Simulate operating conditions by blowing air across the sensor. There should be a frequency swing from the air crossing the wire in the sensor. A normal flow signal will be close to 1200 Hz. If the freqency is not shown, or not proportionate to the air blown across the sensor, the sensor is faulty.
  4. Check for battery positive (B+) on terminal C and ground on terminal B. If voltage or ground is not present, check the circuits back to the PCM for continuity.
  5. If you still receive a proper amount of voltage at the electrical connector and still have a driveability problem, it is probable that the MAF sensor will need to be replaced.
  6. If the sensor and circuits are functional, the PCM may be faulty.

Oxygen

TESTING




WARNING Do not pierce the wires when testing this sensor; this can lead to wiring harness damage. Backprobe the connector to properly read the voltage of the HO2S.Single Wire Sensor

  1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  2. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.
  3. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  4. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
  5. If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
  6. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
  7. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
  8. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

 

Heated Oxygen Sensor

  1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  2. Turn the ignition OFF disengage the H02S harness connector.
  3. Connect a test light between harness terminals A and B. With the ignition switch ON and the engine off, verify that the test light is lit. If the test light is not lit, either the supply voltage to the H02S heater or the ground circuit of the H02S heater is faulty. Check the H02S wiring and the fuse.
  4. Next, connect a high impedance ohmmeter between the H02S terminals of the heating element and verify that the resistance is 11.0-16.0 ohms at 68° F (20° C).
  5. If the H02S heater resistance is not as specified, the H02S may be faulty.
  6. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  7. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) signal wire and battery ground.
  8. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  9. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
  10. If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a faulty fuel pressure regulator. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
  11. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
  12. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
  13. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

 

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