I took my bmw to the deelers and they diagnosed the car as having a faulty mass airflow sensor. i have replaced the sensor and now i have a code of 121. this second code occures after only 35 miles each time i cancel code, but before i replaced the mass airflow sensor it would ocure after about 100 miles each time i canceled it. have i done something wrong when inserting the new mass air flow sensor?
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First two most likely suspects:
1) A big crack in the air tube that runs from the air filter to the throttle body.
2) A failed mass airflow sensor.
You should get a check engine light which will store a trouble code. However, there is no trouble code for "massive air leak in the intake tube", so if that tube has cracked, the code will point to some related sensor or system.
I would remove the air filter and mass airflow sensor and the air tube up to the throttle and the rubber boot that attaches to the throttle body. Check the tubing for cracks that may be letting in excess air that is not being measured. It might be helpful to clean the throttle valve and the mass airflow sensor with mass airflow sensor spray.
Other problems that can cause the symptom you describe are degradation of the crank position sensor or possibly the cam position sensor (cam position sensor error will be most noticeable at idle). If those are causing the problem, as it gets worse it will eventually set a code and light the check engine light.
The p1140/121 code indicates the MAF was not as expected compared with values from IAT, TPS, CTS, IAC command, and RPM. Meaning all THOSE sensors and the IAC solenoid position data have to be working and in normal range for the MAF to test normal and not code. Further, any vacuum leaks will ruin the MAFs accuracy when compared to the others, as will a PCV/CCV problem. If you lobbed a MAF at it already...you have already done the least likely and most expensive part. You really need to look at data and see what the DME sees. Is throttle showing closed at idle, is air temp normal? Is RPM under 725 idling, does it flare idle up and down in 10 seconds. is there any vacuum leakage anywhere? even a small amount can make a code level of difference?. Intakes leak a fair amount. CCV valves much more likely to leak, as are the hoses to it. TPSs dont fail that often but can. IACs usually code when they fail...but they could plug up and not code but have improper air bypass that affects the MAF reading. Throttle cables/adjustment is critical...keep that high on priority list. iATN Tech.
A few classic possibilities here. First your throttle body sensor may be bad. Second your mass airflow sensor may be bad. Third your fuel regulator may be on the blitz. Also, if your oxygen sensor is going bad you may feel a slow response to your acceleration. My best bet is that your mass airflow sensor is going bad. Please have your car diagnosed at the nearest AutoZone for free.