This keeps coming on my Sentra - I clear them, the come back a little while later. MAF dirty? Anyone else have this exact same issue? The codes say "System Too Lean"

Those codes together usually mean a vacuum leak, although they can also be caused by a bad MAF, a front HO2S that's dying but not dead, or bad fuel. The most likely cause on an older Sentra is an intake boot that has hardened and developed a small split.

Thanks VStar - I'll take a look on those. For the MAF, is there a way I can test to see if it's still working with a multimeter?

Forkster wrote: ↑

Wed Feb 24, 2021 11:42 am

Thanks VStar - I'll take a look on those. For the MAF, is there a way I can test to see if it's still working with a multimeter?

No, it's a PWM signal and the ECM calibrates to it, a VOM won't tell you much. But even most cheap OBD scanners will stream it.

Some of my troubleshooting. Voltages seem OK for the O2 sensors - fluctuating between 0.1 and 0.9 V. Mass airflow sensor isn’t jumping around everywhere, when compared to the throttle position moves up nicely.

I have a question (pic 2), this hose I have circled in red, is that supposed to be connected to something or is it just an exhaust hose? It is connected to the airbox.

Forkster wrote: ↑

Sat Mar 06, 2021 2:08 pm

I have a question (pic 2), this hose I have circled in red, is that supposed to be connected to something or is it just an exhaust hose? It is connected to the airbox.

It's the CVT vent hose. No, it doesn't connect to anything.

Thanks Vstar. Ok, I used to ole WD-40 spray&pray to find the vacuum leak - no joy. If I was to connect a smoke machine, what hose can I connect to in the engine (if it’s a vacuum issue).

Forkster wrote: ↑

Sat Mar 06, 2021 2:51 pm

If I was to connect a smoke machine, what hose can I connect to in the engine (if it’s a vacuum issue).

There's an evap service port, but basically anything into the plenum that's un-ported. The booster is convenient, but if there's no joy make sure to smoke the booster itself and check inside the car, diaphragms can leak.

VStar650CL wrote: ↑

Sat Mar 06, 2021 2:58 pm

Forkster wrote: ↑

Sat Mar 06, 2021 2:51 pm

If I was to connect a smoke machine, what hose can I connect to in the engine (if it’s a vacuum issue).

There's an evap service port, but basically anything into the plenum that's un-ported. The booster is convenient, but if there's no joy make sure to smoke the booster itself and check inside the car, diaphragms can leak.

Good point - My smoke machine consists of a nice long transparent tube, and a Montecristo #4.

Ok - smoke test shows no vacuum leaks. Onto next items - MAF sensor?