Hey Folks

I'm chasing a P0171 and in the process of testing sensors. Speaking as more of an idiot than an electrician. The car is running smoothly, just a bit down on power and throwing the too lean code.

I've cleaned the MAF and the entire intake thoroughly, found no vacuum leaks, new air filter, plugs and wires, new fuel filter, still too lean. All OEM / Denso parts. Electrical grounds and fuses are in good shape. This is a "federal emissions" vehicle, so one cat, sensor on either side.

My multi-meter has been giving me some readings pretty close to what Toyota has in the Factory Service Manual. I have been getting a few odd or incorrect readings as well. I have been surfing the "search" function on these forums and how-to videos as well, but I still have a few obvious questions.

Generally when using a multi-meter to read resistance, I have been setting the meter to 200 Ohm. Is that the correct setting?

Ambient temp during this process is about 30 degrees F.

Testing the fuel pump itself, the FSM says 0.2-0.3 at 68F. My multi-meter reads 1.1

A/F sensor (upstream from the cat) should be 0.8-1.4 at 68F. Mine reads 12.1

O2 (downstream from cat) and MAF sensors appear within range.

These numbers lead me to believe my A/F sensor is bad (or is actually an O2 sensor and I didn't look at the part numbers close enough), so ordering another Denso replacement now. Does the reading I get from the Fuel Pump seem acceptable?

While testing the MAF sensor the FSM tells you to check resistance on the harness side of the plug to the MAF sensor. It looks like you're supposed to just shove the multi-meter probes into the back of the plug through the squishy insulation around the wire... This seems like a bad idea, am I wrong?

Thanks for reading, thanks for your patience with the obvious questions.

If you have a '99 Federal emissions, you have O2 sensors for the front and rear. Front AFR sensors are on 99/00 California and all 01/02 trucks.

When measuring very low resistances, you need to check the resistance of your leads by themselves and subtract that from your reading, and even then it is a crapshoot for how accurate you can be. I wouldn't really trust any reading below about 2 ohms unless you really know what you are doing.

Does your OBD-II reader do live data? If so, track short term and long term fuel trims and see if you can find a correlation to engine RPM and load where they are farthest from 0%. A good running (and unmodified) truck will be less than 5% basically everywhere in the RPM/load matrix.

If things are worse at low load, it hints at vacuum leaks, if it is worse at high load, it hints at fuel delivery limitations (fuel pump, filter, injectors, etc.).

-Charlie

Like above you need to look at live data.

750rpm warmed up idle MAF reading and the idle and lightly loaded on a flat road between 40 and 50mph LTFT.


This is a good MAF at idle. Two 97’s had the exact same reading.


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Hey, man!

I'm a newbie, but I think you should double check the info in your Service Manual.

My copy says says 0.2-3.0 at 68F.

Best of luck, and if anybody knows where to find the Open Circuit Relay on a '97, I'd love to have that knowledge.

Woodjunkie, you're correct. The FSM fuel pump high reading does spec to 3.3 ohms. So 1.1 ohms is absolutely correct, and right in the middle of the range. That's the best possible reading. Given the temp, 30 ° F, getting that reading is unusual, but not beyond the realm.

Cold temps tend to give higher than usual readings, but not necessarily out of the range of possible readings. Given the 1.1 ohms that Krommeweg got, I would say the fuel pump is fine.

If he got 12.1 he got for the AF sensor, it MAY be bad. He should remove it, and take it in the house, and read it there, to get a reading within the temp range the FSM specs out. By the same token, if he's getting a reading from an O2 sensor, which he suspects he got, instead of an AF sensor, could it be correct? 12.1 ohms is good for an O2 sensor, according to the FSM. Just a thought.

BTW, 200 ohms is fine for checking the low readings Krommeweg is getting. On many multimeters, that's the lowest range there is.
Just a note on multimeters, too: make certain you remove at least the negative lead from the battery before you ohm out anything, especially is you leave it in-circuit while testing. The slightest current that passes through the meter while testing can blow the meter's ohms portion. Most meters have a fuse protecting the ohms section, but not all do. Something to bear in mind.

Also, yes, you can slide the meter leads into the back of the connector without hurting anything. Alternatively, you can take a very small piece of solid wire, and slide it into the connector's rear, and the touch the meter lead(s) to that. Whatever you use, just make sure it touches the metal pin inside the connector. Not some piece of insulation, or connector plastic. No damage should be done the connector, as long as no excessive force is used at any time.

Good luck to both of you.
Pat☺

Thanks for the thorough response. This is actually a Canadian vehicle, but it looks like in '99 they conformed to the same US Fed standards. When I replaced the exhaust from the Y pipe down it was so corroded that the 02 sensors disintegrated in my hands. The factory system definitely had one cat though.

When I have cleared the code, the issue always comes back within 20 miles and typically under high load. Uphill in 4WH, accelerating to highway speeds, etc. The fuel pump seems fine, and the truck runs smooth, although it's at that mileage where injectors begin to fail.

This process has convinced me to buy a ScanGauge. Looks like it does everything although it doesn't look like it displays real time ST/LT fuel trims... The Innova 3130 looks like a good choice too.

Yeah you're absolutely right. Thanks!

Thanks Pat! I appreciate you taking the time to answer the obvious questions.

Pulled both 02 sensors and brought them inside to test, they're reading within range. Did the carb cleaner test again, still can't find a vacuum leak. Then I pulled and cleaned the MAF again, tested it according to the FSM one more time, now I'm getting an incorrect resistance ( 1 ) and no voltage with it plugged in and the ignition on.
Tomorrow I'm coming home with a hand held diagnostic which reads live data.

Thanks, I appreciate that. You like that 3130 scanner? It's definitely more in my price range than some of the other options.

Both the SGII and SGIII do ST and LT fuel trims. On '99+ you can get the right codes to read trans temp, and on two-cat models it will read the front AFR too (again, with the right PID and math).

I have been fighting a P0420 issue recently and its nice to have a scanner built in at all times for checking/resetting codes...

-Charlie

I have quite a few dedicated scanners.

It is old but I still like it and use it when I need a dedicated scanner. My SG doesn’t read MAF. And I can’t find the PID to program it.

SG’s website has the PID for the LTFT for Toyota and a bunch of others if it doesn’t show up.


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Thanks for all the input folks. I can't say it enough.

I disassembled and cleaned everything between the airbox and the throttle body, cleaned the MAF, new air filter, and reconnected the battery on Monday. Done that exact process at least three times in the last two months.

(Been through three air filters this winter on account of how sh!tty and dirty the paved roads are, not to mention the gravel. Also these Nokion tires are THE BEST STUDDED TIRE EVER MADE but they throw a lot of sand/rocks. The most recent filter was only a month old, but I had a new one on hand, so I swapped it.)

I let it sit for a day and drove back into town today. No codes so far. Spent $200 on a Innova 3100 at my local NAPA, hooked it up, ran errands around town and drove home. Still no codes. ~40 miles total, hills, highway, gravel, 4WD, same as always. Since this problem cropped up, the Check Engine light has always come back on before I complete the trip, within 20 miles. Not today Satan!

LTFT at cold idle was -5%, -3.9% at warm idle. 0% around 1900rpm and 60mph. It never exceeded the range between -5% and 3%.

MAF was right around 0.5 lb/min at idle.

All other values seemed within range... and that fuel trim fluctuation seems pretty darn minimal.

...so I guess I'll be up here, waiting for something to fail again.

If your fuel trims are back down to <+/-5%, you are not going to get any lean/rich codes. Looks like you are good for now.

-Charlie

I chased a lean code and a warm start issue for a year. Ended up being the fuel pump. See attached video of my 4Runner.

https://www.youtube.com/watch?si=1xSjIcuSK2x8T4hR&v=uCriDDdw-V4&feature=youtu.be