OBD code P0171

I have a Mazda 1999 Protege with the Check Engine light ON, and indicating OBD code P0171(System too lean). The dealer replaced the mass air flow sensor. The Check Engine light came on again, and the OBD code is again P0171. Now the dealer says the car needs “brake boosters”!!! Given the OBD code, the air flow sensor made sense to me, but BRAKES??? Is there ANY WAY the brakes could cause OBD code P0171, or am I about to be taken for a ride (again)?

If the diaphram in the brake booster has a leak it can pull excess air into the engine which would cause this code. But if it were sucking in that much air, I would think there would be a hissing sound and/or a change in how the brake pedal felt.

If you don’t sense any of this happening, then the brake booster would be the last thing to suspect. There are many other things that can cause this code. And the MAF sensor is one of them. But if that didn’t fix it, there other things on that diagnostic flow chart that can cause this code. And those would be the things to investigate before the brake booster.


I can’t see any way that brake boosters would cause this code. The normal solution to this is to clean the MAF sensor…in this case it was replaced, so obviously that wasn’t the problem.

My next guess would be a vacuum leak. All hoses should be inspected; it wouldn’t surprise me if one or more of them are cracked and leaking after 10 years. Another possibility is a dirty fuel filter. Has it ever been changed?

Check this site out - http://www.aa1car.com/library/ford_lean_codes.htm

It will help you as you go to talk to the mechanic - but Tester & budd are probably right, and you may want a whole different mechanic. Although the condition your mechanic implies is a vacuum leak (the brake booster is part of the vacuum system) there’s a lot to do before you’d ever end up there. Aside from that you would have other symptoms of a bad booster as noted by Tester.

Thank you! I have not noticed those symptoms, but will check for them before proceeding.

Thank you! Your suggestions match what I found at obd-codes.com

Thank you! Great website…very informative.

It’s easy enough to test if the brake booster is leaking vacuum (which would allow extra air to go into the engine intake manifold): To test, disconnect the vacuum hose from the intake manifold that goes to the brake booster, and attach a hand operated vacuum pump (with gauge) to the hose. Use the vacuum hand pump and attempt to pump the brake booster to 20 inches of vacuum. Use a 4’ long hose (to allow reach), or have someone depress the brake pedal to see if the vacuum falls off.
If the brake booster doesn’t hold 20 inches of vacuum, replace the brake booster.