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Juan Crespo
Juan Crespo, Tech Trainer
Category: Car
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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Will a bad MAF sensor always cause a CEL? EGR is clean. Still

Customer Question

will a bad MAF sensor always cause a CEL? EGR is clean. Still have p401 code. Hesitation on acceleration when cold
Submitted: 2 years ago.
Category: Car
Customer: replied 2 years ago.
2001 isuzu trooper 3.5L. I have cleaned the EGR valve and all connecting tubes and throttle body. CEL is on with 401 code. When cold the truck hesistates on initial acceleration then smoothes out in the upper RPM range or when operating temps come up.
Expert:  Juan Crespo replied 2 years ago.
Hi there. Welcome to our site. Everytime the computer commands the EGR open, it expects to see the appropriate change in manifold pressure. When it doesn't get what it expects in two consecutive "trips", it takes a snap shot of a few engine parameters, turns on the check engine light and stores code P401. Taking that into consideration, the code could be caused instead by lower than normal fuel pressure, misfires or restricted exhaust. Looking at the freeze frame data that was taken by the computer when it stored the code will help us see what was going on with the engine at the time of the failure. Can you share that information?
Customer: replied 2 years ago.
I do not have that info. Code was pulled with a basic scanner at a parts store
Expert:  Juan Crespo replied 2 years ago.
In that case, do you have any other type of data that would help me help you solve this issue? For example; what's manifold vacuum at idle and at 2000 RPM when engine cold and then when warm? The same for fuel pressure, what is it when cold, warm? If you can't measure exhaust back pressure, you could remove the two upstream O2 sensors from the exhaust (the ones closer to the engine), secure them and keep them connected to the harness; then drive the vehicle to see if the hesitation is still there.
Expert:  Juan Crespo replied 2 years ago.
Hi again. Just checking to see if the information I provided helped you solve your issue. If it did, then kindly accept and rate my answer now. If it didn't, please reply so we can keep working.
Best Regards.