Chasing the P0171 Engine Light

April 23, 2018.

I was ready for adventure — running solo, I was returning to Death Valley, having only just wet my appetite on our first trip back in January. With 2500 miles to go, I knew I had a lot of driving ahead — but I was ready — or so I thought — when, less than 50 miles in, the Check Engine Light (CEL) came on.

I’ve covered the details of the first hours of diagnosis in my Back to Death Valley trip report, so if you haven’t read that (and are interested), I’d recommend starting there. The plan here is to go into what happened when I returned — the process of chasing the P0171 source, and getting it fixed.

It’s long, and wordy, and follows my learning process. If you already know everything here, sorry about that!

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Upon my return, I started with a bunch of research into what can cause the P0171 code on a 96–04 Tacoma, which is set when Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT) exceed 15% for an extended period of time (over the course of two trips). It’s not a small list, and I’m sure that even what I uncovered isn’t complete:

  • Dirty or malfunctioning mass airflow sensor (MAF)
  • Vacuum leak
  • Malfunctioning air/fuel ratio sensor (AF)
  • Fuel injector clogged
  • Fuel filter clogged
  • Fuel pump weak
  • Malfunctioning positive crankcase ventilation valve (PCV )
  • Engine coolant temp sensor malfunction

I decided that I’d try a multi-pronged approach, largely centered around what seem to be the most common culprits — a vacuum leak, the MAF, and the AF sensor. I ordered a new Denso (OEM supplier) MAF sensor (197–6020) and AF sensor (234–9003), as well as a new OEM fuel filter (23300–62010), so that they’d arrive as soon as possible.

Having already cleaned the MAF, I started by looking for a vacuum leak. I’d done this by spraying starting fluid in the engine bay in hopes of hearing the engine rev while on my trip, but I figured a smoke test was in order. So I grabbed an old paint can, a length of hose, and an air valve to construct my own personal smoker.

Then, I built a fire. My plan was to sprinkle a bit of olive oil on them inside the paint can to generate smoke, and then to push the smoke into the system with a bit of air pressure (just a couple psi — I didn’t want to blow any hoses off in the process).

I was generating lots of smoke, so I removed the air box (filter + MAF) and covered the intake with a blue glove. Then I hooked up the clear hose and sent the smoke into the system. There was plenty of smoke getting in (and coming out once I took off the blue glove), but as I’d found with the starting fluid, I had no leaks.

With that (as unlikely as it was, since it seems that P0171 is most commonly caused by a vacuum leak) I was confident that I could start focusing elsewhere. Taking stock of the data I had seemed to make the most sense, and so I pulled up the screenshots I’d taken when I was on the road to Death Valley.

Fuel Trim Data At Idle

Fuel Trim Data Under Load

A bit more reading and YouTube research suggested that I should…

…you’ve read this far, I’m sure you’re curious. Read the rest of Chasing the P0171 engine light at

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