Upstream vs downstream O2 sensors

I bought a bosch AFR sensor when my stock one died and have had no issues so far, they have a much better design than oem IMO. The rear sensor, like others have said, doesn't matter, only the front one. Before buying one you'll need to determine what you truck has, a narrowband sensor or a wideband. 99-04 with automatics have widebands (manuals to I believe?), 98-95 with manuals and automatics have narrow band sensors. The sensor you linked WILL work, I know because I have it on my truck.

 

That is true for your model year. Later years' sensors are distinctly different. The upstream air/fuel sensor has fewer ports for sampling the exhaust and also the shorter wire. Denso's description of the two sensors is exactly the same though, at least on the RockAuto web site. And of course the upstream sensor is more expensive.

 

well shucks... the upstream O2 sensor did not fix the problem (p0171). Waste of 50 bucks. Can't find any vacuum leaks, at least obvious ones.

Cleaned the MAF once again (This time better..inside wires i forgot). Reset the code. We'll see.

 

I keep getting that code as well (P0171). The MAFS is clean, and I cannot find any vacuum leaks.
Is there a way to see if it's a bad MAFS?

 

If you're running poor at idle and light load it won't have anything to do with the MAF. The computer reads the oxygen sensors output alone and adjust fuel to achieve 14.7:1 afr at any condition under approximately 80% engine load. Lean doesnt necessarily mean you have too much oxygen but that you have too little fuel, and p0171 means the computer is unable to correct the air:fuel ratio within its set parameters. Could have more to do with your fuel system, clogs, tired pump, etc. Start with the cheap stuff, fuel filter and maybe a bottle of injector cleaner. Good luck.

 

@Clay_916 Thanks.

 

This is old but I'm loving the signature so it was worth the quote.

 

ive recently been visited by the p0171 code also.I noticed knocking under load kinda like pre-ignition,i changed the air filter, cleaned the maf sensor and throttle body.The truck now has a perfectly smooth idle but no other changes and light came back on.I changed the fuel filter and wow, what a difference that made.The truck pulls up hills it couldn't do before and feels so much better but the code came back again.lol..Im going to try checking all vacuum lines for leaks and maybe zip tie them all.

 

Jay, if you can, use a obd II scanner and do live data and read your fuel trim (long/short) numbers. They say + or - 10 on both trims your good. If your reading like +20 to +30 at idle and it drops closer to 0 when you raise the rpm's on your long trim. It points to a vacuum leak. Or spray some carb/brake cleaner around your intake and vac lines and see if you get a change in rpm's. You may already know that. Just trying to help. I am going through same thing but my long trim number is -18 at idle. Believe that rules out vacuum leak. Fuel pressure is good at 43 psi so, I rule out pump / filter. Installed new Denso a/f sensor. I am now thinking dirty/clogged sensor or sensors. My symtoms are, when you quick hit the gas pedal, it cuts out before it picks up. You do it slow it's fine.

 

Hey all, I don't mean to highjack this post, but does anyone know where I can find a right up on how to replace the upstream o2 sensors on a gen2?

 

It's just two bolts and a connector. You can try here I guess
https://www.tacomaworld.com/forums/2nd-gen-tacomas-2005-2015.5/

 

Its easy. Just a bitch when you gotta dc the front 02. My 08 taco bank 1 is the passenger. Bank 2 is driver. I got ntk 02 and ill see if i can clear my cel. 2 sensors. Back for cats. Front by headers for afr. follow line. Dc unplug.

 

I just fixed this code on mine it was a busted vacuum hose on the upper intake manifold. I cleaned my maf as well. From what i know a p0171 is usually a bad or dirty maf, a vacuum leak bad, o2 sensor, exhaust leak, or clogged fuel filter. Like stated before the bank 1 o2 is seeing elevated oxygen levels, this tells the ecu the system is seeing more air than it can supply fuel within its parameters so it throws a lean code. I have heard in rare cases a bad fuel pump was the culprit. Also check for exhaust pipe leaks before the upper o2 it will make it read lean as well if air is getting in.

 

I will check the hoses.

 

That's what I thought, but they're different, and I found out the hard way.

Upstream air-fuel sensor is DENSO -

$153 234-9002 Air Fuel Ratio Sensor

234-9002 (w/Auto Trans), -9001 (w/ manual trans)

It's flange bolted to the exhaust ahead of the cat, short pigtail, plugs into the harness under the truck.

Downstream oxygen sensor is DENSO -

$61 234-4189 Oxygen Sensor

234-4189

Sensor looks the same, same flange, fastens on to the exhaust in the same way, behind the cat. More wire on this one, run up through a rubber gusset/boot on the floor, up under the carpet, where it plugs in under the passenger seat.

They look like the same thing, but they are not, and the OBD computer knows it.

Truck is a 2002 Tacoma 2.4L DOHC, other models may vary.

 

I second the small vacuum leak diagnosis. The 3.4 has some “hidden” vacuum hoses. There is a vacuum Y in the back of the block under the intake that feeds air into the injector cups that everyone seems to miss. The brake booster is another source of difficult to locate vacuum leak.