Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Black, sooty tailpipe

2000 Toyota Tacoma, 4 cylinder. 125,000 miles. Everything has been working fine. However, I decided I might as well replace the oxygen sensors fore and aft on the tailpipe. Old ones were fine, but 10 years old and over 100,000 miles. I replaced them with factory units. Now the tailpipe shows black soot and mileage stinks. What happened?

You used incompatable o2 sensors…Where’d u get them…I am certain they werent from Toyotie

When a car runs well, the only thing you change is the oil, filter and the coolant at that mileage. Or the spark plugs if needed.

If the oxygen sesors start acting up you will know it.

Since you’re stuck with a strange situation, put the old rear one back in and give the computer two days to figure things out. If the soot and smell goes away, you can put the new rear one back in and things might straighten out.

I don’t know this stuff for certain but I did read about the O2 sensors in a parallel universe.

My way to buy the sensors would be to remove the old one and bring it to the dealer and they would use the serial number to get the right one. Parts places will sell you anything from a wrong page in the catalog or computer. The drawback is that the dealer may be expensive.


The aft O2 sensor just double-checks the catalytic converter operation. The fore O2 is the one to replace with the original, as it is used to control the fuel mix. The fuel mix on this truck is way off, leading to a seriously rich condition.

No need to read about it on Earth-2. Plenty of material here.

You might have to get the code reader and erase all codes. The computer is using the old parameters and can’t adjust the fuel mixture. Once the monitors are reset to “not ready” they might be able to straighten everything out.

Might is kind of a big word here. As BustedKnuckles so kindly shouted out; there is a seriously rich condition and putting the original front sensor back in will straighten that out.

I reasoned that the computer would recognize the input from the old aft sensor and then would adjust for the front one. The computer would probably say “good luck with that”.

I see that you replaced them with factory units. At least you were trying.

I have noted that the post converter oxygen sensor has no effect on the air-fuel ratio and is used to monitor the efficiency of the catalytic converter. It does seem that the forward sensor is out of whack.

I got up this morning and realized that I should get back to you because there may be a specific thing you had to do after changing the sensors. There isn’t one on my truck which is a GMC Sierra 4.8 but I don’t have a Toyota book. The Haynes manual gives a pretty good description of what the O2 sensors do.

So, you may not get anywhere by erasing all codes but I don’t mind experimenting on YOUR truck.

Busted is correct… In fact since you had no issues prior to messing with the O2 sensors before…I would simply re-install the old ones… You WILL KNOW when they go bad afterall. Right now you are “fixing” an issue that isnt there…

I got the correct ones, from Toyota. Both Densco and with the same part numbers on them as the old ones. On the 2000 model, both are identical models. Same one front and rear. Should I assume the new one/s are defective?

You should only assume the black sooty buildup is from the engine running too rich. Find out why before you fry your catalytic converter.

Have you checked some other tailpipes to compare?? Most tailpipes are black and sooty…

If it’s not broke, fix it until it is…

When I buy a used car a black sooty tailpipe is an automatic walk away.