Accelarating problam. diognized code P1128 - Fuel system lean part load. What are the possible causes ?

Hi there:
DTC P1128 is "Long Term Fuel Trim Too Lean"
This means the air/fuel mixture is leaner than the HO2S can compensate for.


This is most often caused by a vacuum leak. It can also be caused by a clogged fuel filter, unmetered air entering the air intake system, leaking exhaust system, canister purge system not operating or a weak fuel pump.


Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using FixYa, have a nice day.

CODE P1128 LONG TERM FUEL TRIM ADD BANK 1 SYSTEM TOO LEAN CONDITIONS.SOUND LIKE YOU HAVE WEAK FUEL PRESSURE.FUEL PRESSURE SHOULD BE WITH NEW FUEL FILTER 36 PSI FOR THE ENGINE CODE AMU. AND FOR ENGINE CODE AEB 50 - 58 FAULTY MAF SENSOR.CLEAN FUEL INJECTORS THEY COULD BE CLOGGED CAUSE LEAN FUEL CONDITIONS,FAULTY SECONDARY AIR INJECTION SYSTEM COMBINATION VALVE STUCK OPEN OR SECONDARY AIR INJECTION SYSTEM HAS A ELECTRICAL SHORT OR ECM CONTROL FAULT.CHECK FOR INTAKE MANIFOLD VACUUM LEAKS. CHECK ALL VACUUM HOSES ON THE INTAKE.

code P1128 02 sensor bank 1 lean
code P1129 02 sensor bank 1 lean to limit
code P1133 02 sensor bank 2 lean
code P1134 02 sensor bank 2 lean to limit

fuel system is lean.

P1124 = LONG TERM FUEL TRIM ADDITIVE, AIR SYSTEM TOO LEAN
P1128 = LONG TERM FUEL MULTIPLICATIVE, SYSTEN TOO LEAN

I would start with checking fuel pressure, possible the pump or regulator could be faulty.

A quick check on the net indicates MAF sensor or intake leak:

P1128 - Long Term Fuel Trim Multiplicative System Too Lean.
The ECM will set a code and the MIL will turn on if the long-term fuel trim value goes from 10% to 15% lean for 30 seconds during two driving cycles when the following conditions are met:
1.- Engine loads exceeds 1.8 milliseconds.
2.- Engine coolant temp. is above 158F (70C).
3.- Canister purge system is not operating.
4.- Mass air flow is less than or equal to 5.5grams per second.
5.- Engine speed is below 1000RPM.

Did you erase the codes? Did you put in a new MAF sensor or a junkyard sensor? Check the intakes for cracks or holes?

Good luck.

Remove the MAF sensor and carefully clean the sensing diodes with some solvent and a q-tip. These codes are commonly caused on this car by a dirty MAF that does not see the air entering the engine so fuel is not added by the computer. Then the computer sees that the mixture is lean by looking at the exhaust gas oxygen (sensor) content and it sets a code because it can't trim fuel delivery to the correct level as the MAF signal has more authority.

P1128 - Long Term Fuel Trim Multiplicative System Too Lean.
The ECM will set a code and the MIL will turn on if the long-term fuel trim value goes from 10% to 15% lean for 30 seconds during two driving cycles when the following conditions are met:
1.- Engine loads exceeds 1.8 milliseconds.
2.- Engine coolant temp. is above 158F (70C).
3.- Canister purge system is not operating.
4.- Mass air flow is less than or equal to 5.5grams per second.
5.- Engine speed is below 1000RPM.

Please Rate Me =]

Trouble Code: P0134


HO2S (Bank 1 Sensor 1) Circuit No Activity

Possible Causes:


Leaks present in the exhaust manifold or exhaust pipes

HO2S signal wire and ground wire crossed in connector (voltage jumps)

HO2S element is fuel contaminated or has failed

ECM has failed



Trouble Code: P0172


Fuel System Too Rich (Cylinder Bank 1)

Possible Causes:


Camshaft timing is incorrect, or the engine has an oil overfill condition

EVAP vapor recovery system failure (may be pulling vacuum)

Fuel pressure regulator is damaged or leaking

HO2S element is contaminated with alcohol or water

MAF or MAP sensor values are incorrect or out-of-range

One of more fuel injectors is leaking



Trouble Code: P1128


Long Term Fuel Trim Add. Air. Bank 1 System Too Lean

Possible Causes:


Fuel pressure is too low or fuel quantity supplied is too low

Fuel filter faulty

Transfer fuel pump has failed

Fuel injector is faulty (sticking or not opening)

Engine speed (RPM) sensor is faulty

MAF sensor circuit open

MAF sensor circuit shorted to ground

Air leak in the manifold

Secondary air injection system combi-valve stuck open

Secondary air injection system electrical short

ECM has failed

intake manifold gasket leak vacuum leaks, a defective MAF sensor, low fuel pressure under a load or a defective O2 sensor.