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Pavlin Koev
Category: Toyota
Satisfied Customers: 3485
Experience:  20 years hands on,Experience cars and trucks, gas and diesel
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Toyota 4Runner SR5: I have a 2003 4Runner that has a chronic

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I have a 2003 4Runner that has a chronic problem with A/F sensors failing over the last 4 or 5 years. I have replaced the sensor 5 or 6 times on Bank 1 and twice on Bank 2. Then engine code is usually P2238 (P2241), but sometimes P0171 and/or P0174. I have noticed in the last few that there is fluid on the connectors (both sides), but no sign of oil leaking near the connector. I have read about a couple of instances of transmission fluid wicking through the wire harness and this sounds like what could be happening with my vehicle.
What are the possible sources if fluid is wicking through the wire harness? I’m assuming it is a sensor on the harness and somehow getting into a ground or power wire that is common to multiple sensors. The fluid appears to be dark black. Is there any way to tell if it is engine oil, transmission fluid, or another fluid? Should it stop if I change the sensor at the source? Or cut the ground wire and put a solid connector or solder inline to prevent the wicking?
Thank you for posting! If you need more assistance please REPLY otherwise less then perfect feedback from you means I don't get paid.
Question for you here: what code #'s are stored right now? When you are replacing sensors are these ones that you are purchasing from the dealer or are these aftermarket from a parts store?
Customer: replied 5 years ago.
I most recently had P0171 and P0174 last week. I replaced both A/F sensors and the check engine light went out briefly, but came back. I then replaced the downstream oxygen sensor on bank 2 and cleared the codes. The light has not come back since (6 days and several hundred miles). So no current codes.

I have always replaced the A/F sensors with dealer replacements until last week. Now the bank 1 A/F and bank 2 downstream are Bosch. The bank 2 A/F was replaced last week with a dealer replacement part.
Ok but no codes currently correct? Do you have any pictures of this fluid you speak of?
Customer: replied 5 years ago.
I can get some this evening.
Ok thanks
Customer: replied 5 years ago.
I don't have any codes now, but based on past experience I am expecting to have have them show up again within a few months. Attached are pictures of the connectors from the three sensors I replaced last week. Bank 1 Sensor 1 (A/F sensor) has the dark fluid. There is a picture of the connector and a picture showing how much oil drained out into the plastic bag after I removed it. The other two pictures are of the Bank 2 Sensor 1 (A/F sensor) and Bank 2 Sensor 2 (oxygen sensor). They appear to have a film of a fairly clear fluid.
these are weather sealed connectors, no fluid of any type should be able to get in regardless of if it is submerged or pressure the orange weather seal in place in the sensor connectors?
Customer: replied 5 years ago.
The orange weather seals around the four wires appear to be in place. The four wires look/feel fairly clean as does the grey connector housing near the wires.
I’m a moderator for this topic. Your Professional has opted out and I wonder whether you’re still waiting for an answer. If you are, please let me know and I will do my best to find another Professional to assist you right away. If not, feel free to let me know and I will cancel this question for you. Thank you!
Customer: replied 5 years ago.
I am still waiting for an answer. An answer posting between "Toyota Tom" and a customer had similar symptoms and that discussion lead to me posting my problem. Can you direct the question to Toyota Tom, or open it to the general expert community? Thanks.

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Customer: replied 4 years ago.
Please keep the question open.

Different Expert here!

Since you are currently getting a P0171 and P0174 codes, this needs to be addressed first. These codes are usually due to a bad mass air flow sensor. It might just be that simple. Just be sure there are no air or vacuum leaks under the hood and that the fuel system and regulator check out ok. If so, I would replace the mass air flow sensor, clear and the codes and go from there.

Let me know how it goes!
Customer: replied 4 years ago.
I don't have any codes currently. As I said above, once I changed out the oxygen sensor, the codes have not returned.
I previously cleaned the mass air flow sensor and checked the resistance, which was in the reasonable range. I also have had the intake manifold gaskets checked and they found no leaks.

You need to replace your mass air flow sensor and clear the memory in the computer.

Customer: replied 4 years ago.
I do not have any codes and have not since I replaced the oxygen sensor a couple weeks ago (and cleared the codes). I will need some better justification before I replace the mass air flow sensor. Please take another look at my posted questions, which are in related to the fluid in the connector and focus your response on that area.

I read over everything again.

Is your question still "where is this fluid coming from"?
Customer: replied 4 years ago.
That I will not be able to tell you without looking at the vehicle.

You will need to clean the engine off using degreaser and see where the oil or fluid is dripping from as the engine is running.
Customer: replied 4 years ago.
Relist: Answer quality.
Expert is not answering my posted question

Hi and Welcome to Just Answer! My name is XXXXX XXXXX I will do my best to help.

this looks like engine oil to me. the only way oil to get there is leak from engine valve cover gasket and creping or dripping down. Have you tried using a mirror and a flash light for locating a leak from top, which can be dripping on the connector and entering connector?

Customer: replied 4 years ago.
I checked very thoroughly, visually and by feel, around the area of the connector, on the connector, on the plastic "over wrap" on the wire bundle going to the connector on the wiring harness, and inside the over wrap on the insulation of the wires going to the connector. I cannot find any oil or fluid. In general, the areas are fairly clean. I did unplug the connector on the A/F sensor that I installed two weeks ago and see evidence of more oil inside the connector.
The only explanation I can come up with is that the oil is wicking through a wire in the wiring harness. The quote below from an answer to another customer (292 days ago) by "Toyota Tom", an expert Just Answer, leads me believe this is possible:
"What we've seen is that on the v6 4runners that were the early era of that body style, transmission fluid will actually wick up from the transmission through the wire harness and leak inside the ecu"
Hello, If you have done thorough inspection and did not find any oil leaks, then this is the only option that has left, even though it is hard to believe that oil did fill up the computer first and then moved on to other wiring. You should disconnect Engine computer and inspect it so this can be confirmed. White wire on A/F sensor goes to A/F hearer relay-the other 3 wires are going to ECU connector E5- pins 6/ red blue wire pin22 pink wire pin30/blue wire.

Access ECM located behind Glove compartment. Remove computer from vehicle and remove screws to open computer box. you can examine for presence of oil. examine connector E5 first while disconnecting it for presence of oil. This is same connector which is used for transmission connection.

If ECU is full of oil but it is working fine, then there is no need to replace it. you can spray wash circuit board with brake cleaner and blow dry with compressed air. wiring you will have to replace and Solenoid wiring connector.graphic

Here I am sending you connector view. Tranny oil pan has to be removed for this operation. I suggest you replace tranny filter at same time, if you end up doing the work.

after oil pan is installed, you need 2.0L of tranny fluid to top up transmission oil. use ATF Type T-IV fluidgraphic

Customer: replied 4 years ago.
I unplugged the E5 connector and found fluid on it and every connector below it (E6, E7, and E8). No fluid above it on E4. I will check the computer tomorrow and let you know what I find. On the part of the harness that goes to the transmission, do you know if there is a connector that can be disconnected and checked before taking the tranny oil pan off?

I have sent you a picture of oil pan connector.

please click H E R E to see oil pan removal procedures and wiring colors.

Category: Toyota
Satisfied Customers: 3485
Experience: 20 years hands on,Experience cars and trucks, gas and diesel
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Thank you for the accept and Good rating!


Regards Pavlin!

Customer: replied 2 years ago.

Just wanted to post an update:

I think I have a solution to the A/F sensors failing. I am now pretty confident that the problem was transmission fluid wicking through the wiring harness from the transmission sensor to A/F sensors (including between connectors in the ECM behind the glove box). I replaced the failed A/F sensors with new sensors, but before I installed them I spliced in a piece of solid wire in each of the four conductors. The theory was that this would block the transmission fluid that was wicking through the braided wire. I did this about 1.5 years ago and have not had a problem since. Prior to this fix, the A/F sensors were failing in just a few weeks after installing. I also preformed this fix to one of the downstream O2 sensors that failed. I used high quality sealed but splices to splice in the solid wire segments (

great, thanks for the info.
you can also check the transmission ventilation if not plugged, as this will cause high pressure in the tranny case.
Regards Pavlin!