Check Engine light... AGAIN! (Mazda6s)

Check Engine light, again. 2003 Mazda 6s, 151k miles, single owner. A couple weeks ago, my check engine light came on, code was read as P0174. Per the lookup, the definition is “Lean air/fuel ration bank 2” and this means one of four things:

Low fuel pressure, “large vacuum leak”, dirty/defective MAF sensor, or “engine mechanical condition”. There was no change to how the car was responding to the gas pedal. According to the auto parts guy, the only one of those that would not effect the responsiveness is a dirty MAF sensor, so I got a can of cleaner and used it.

Not only did the check engine light fail to clear, a second code has now popped, P0171. Per the lookup, this one’s definition is “fuel trim bank one condition” with four items to check:

“If bank one and two codes set together suspect fuel pressure or MAF sensor”, Oxygen sensor defective, ignition misfire-repair, fuel injector problem.

There is still no noticeable change in responsiveness to the gas pedal, but now there is an intermittent roughness at idle, and sometimes on start up it’ll idle at ~1.2k RPM instead of the normal 500~700. Googling around indicated I should check my fuel consumption and it is within my expected range.

Since I just cleaned the MAF, it’s not dirty, and engine response is still good so my ill-informed logic (ie, I could be completely wrong) tells me that it’s not low fuel pressure. Obviously, the MAF could have died and I have no real way to determine that without replacing it. Google searching also says these two codes together could indicate a vacuum leak. If there was a vacuum leak, would there be any noticeable symptoms?

I am, for all intents and purposes, flat broke. I’m trying to determine if this is something I can attempt to fix myself with no real mechanics tools and no way to lift the car to get under the engine or if it’s outside my scope, try to narrow it down before bringing it to the shop.

The performance has started to take a hit now. I’m going to buy a replacement MAF, any suggestions on an aftermarket brand to either use or stay away from?

Before replacing the MAF sensor, inspect the intake hose from the MAF sensor to the throttle body for leaks. If no leaks are found, take the MAF sensor cleaner and while the engine is idling, spray the MAF sensor cleaner in short spirts around the throttle body where at attaches to the intake runner and all other intake seams after the throttle body. If the engine idle speed changes you’ve found the intake leak.


I’m pretty sure my 2003 Protege5 produced the same code.

Some guys working at Autozone recommended a fuel system cleaner as a cheap first step. An independent mechanic told me that this is often a problem with the O2 sensor.

I tried to fuel system cleaner. About the same time, I took a look at the MAF sensor. In the process, the air intake hose cracked in half. Some of the rubber was dry and cracking. I replaced the hose and voila! No more check engine light.

Technically I don’t know it was the hose, though I’m pretty sure it was.

Try a cleaning of the MAF, first also.

@ST1168 The torn air intake hose caused the code in your case

Torn air intake hose = unmetered air

Unmetered air = P0174 lean code

Unmetered air is often, but not always, the cause of lean codes

… and once again, this site has failed to alert me of replies.

Considering the car’s recent history and the fact that I received a $2,000 certificate from/to a local Chevy dealer towards anything on the lot, last Thursday, I wound up getting rid of the Mazda6 (10.5 years old, 151.5k miles) and I got a '10 Ford Focus. Even with two codes on the check engine light and visible body rust, I got above KBB value for the trade-in. For the past six months or so, I’ve been having to through anywhere between $100 and $1500 at it every 4~6 weeks.

Before that, I did wind up replacing the MAF myself, Mazda wanted $250 for one, I got a reform from Autozone for $80 instead. That’s when I noticed that one of the screw holes for the MAF was stripped, meaning the module wouldn’t seal. And while looking at how bad that was, the other one stripped… so rather than replacing the airbox top, I got wider screws and secured the MAF back down. The check engine light still failed to clear after a reasonable amount of time, and the engine roughness continued to slowly worsen.

Even if, technically, this could have been repaired with a new use-accessible hose or something, I am happy with this decision. The mileage on the Mazda6 was never stellar to begin with, and has dropped significantly since new, so much that over the past year or so, I’ve had to curtail some activities due to the cost of just getting there and back. I haven’t yet done a real-life MPG test, but this Focus’s city MPG rating is close to what that Mzda6 was real-life getting on the highway.

Too late now but the codes indicate a vacuum leak and the high and sometimes rough idle confirm that it had a vacuum leak.