New exhaust now throwing Code 28 on 1994. (1 Viewer)

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Twin Ports of Superior, WI and Duluth, MN
I just replaced my entire exhaust system this weekend and now I am getting Code 28 which says it has to do with the No. 2 O2 sensor. I wasn't having this issue prior to the new exhaust.

The only thing that I did that may have caused the problem was to clean off the old O2 sensors with brake cleaner prior to re-installing them into new Toyota downpipes.

Can I test the O2 sensor to see if it's working or am I just looking at buying new sensors?

This is on a 1994 with OBDI
 
did you unplug them? I had that code on my 94. Replaced with new OEM O2 and still had them from time to time. I cleaned the O2 plugs, both male and femail ends really well with electrical cleaner and a small something to scrub with. Code never came back after several years. I eventually moved the sensors to where the EGR is in the exhaust manifold to help keep the electrical plugs cleaner.
 
Did you try a drive on the freeway to really heat things up and "burn out the gunk"? Not sure if its the solution but with a fully new exhaust and cleaned out O2 sensors it can't hurt to burn off any residual oils or crap in the system.

My philosophy with codes is to reset them a few times and if they keep coming back its an actual issue vs an anomaly.
 
I didn't have a chance to take it for a long drive, but I can do that later today.

I did try to reset the system and the CEL just stays on.

I am going to try to clean the wire fittings later to see if there isn't something that I can do to get the code to go away. I will let you guys know if I get anywhere.
 
I had a code 26 a few weeks ago. I'm not really convinced it's gone, but I had two things contributing:

  • A tank with the techron additive (like the bottle you pour in).
  • I pulled my plugs and one of them had oil, but not at the spark area (in the cylinder) and not up high where the plug wire connects to the plug.

I changed the plug and am on a new tank of fuel, no more more rough idle or code 26. Time will tell.
 
I had a code 26 a few weeks ago. I'm not really convinced it's gone, but I had two things contributing:

  • A tank with the techron additive (like the bottle you pour in).
  • I pulled my plugs and one of them had oil, but not at the spark area (in the cylinder) and not up high where the plug wire connects to the plug.

I changed the plug and am on a new tank of fuel, no more more rough idle or code 26. Time will tell.

Personally i have never liked any of the additives.Just dont see then need for them and i see potential problems.
 
Personally i have never liked any of the additives.Just dont see then need for them and i see potential problems.

I'm pretty much of the same mind, but I've flexed a bit considering that gas from a pump these days has ethanol in it so we're already putting additives in. With diesels we'd put in the anti-microbial stuff, and for gas that's going to sit around awhile it was sta-bil.

I also felt like if there's a problem that a few ounces of techron was going to unearth than it was a real problem anyway that was waiting to pop up so might as well get it out of the way.

I'll probably just stick to chevron gas which has it diluted anyway.
 
Costco advertises that its gas contains 4x the minimum required fuel cleaners. Chevron puts in Techron. Other put in whatever they want for additives. I don't really know if you can actually buy PLAIN GASOLINE anymore. Arizona requires all gas in major metro stations have 10% ethanol for the winter months. Cant get away from it.
 
OK, I had a chance to dink around with this again today.

I cleaned the terminals on both fittings but the code says it's the No.2 O2 sensor and used dialectic grease on them.

I tried taking the No.2 sensor out and cleaned it again with MAF cleaner and blasted air into it in case there was anything else in there.

Still no luck, so I tried putting pressure on the tailpipe to detect any leaks in the new exhaust and there were none that I can find.

I guess I am on to trying to test the sensor itself somehow. I looked around online and there are instructions to test a sensor, but need to know which wires are in play and I can't figure that out by using the FSM or the FWD(I may not be looking in the right spot). I will admit that electrical work is my weakest skill set, so any advice would be appreciated.

I can't even clear the code. As soon as I start the truck, it throws the CEL almost immediately.

Any ideas?
 
Sounds to me it is the sensor itself. You ecan test the resistance of the heater circuit with an Ohm meter, but to test the signal of the sensor itself requires a more advanced tool to measure an oscillating /step signal. Since it comes on automatically, it's probably the heater circuit throwing a short or infinite resistance.... Or it could be the wiring to the sensor. I don't have the FSM handy but it should tell you which prongs to check.
 
Looks like terminal B+ and HT on the O2 sensor harness... Ohm reading at room temp should be 11-16 ohm
 
Sounds to me it is the sensor itself. You ecan test the resistance of the heater circuit with an Ohm meter, but to test the signal of the sensor itself requires a more advanced tool to measure an oscillating /step signal. Since it comes on automatically, it's probably the heater circuit throwing a short or infinite resistance.... Or it could be the wiring to the sensor. I don't have the FSM handy but it should tell you which prongs to check.


Thanks! I can't find a diagram for the prongs and I am looking at the page now. I am kinda confused by the whole thing to be honest. I may just try a new sensor and see if that does it.
 
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Here's the info on running diagnostics on the o2 sensors. Should be for a '94 OBDI, which is what I have.
 
The only thing that I did that may have caused the problem was to clean off the old O2 sensors with brake cleaner prior to re-installing them into new Toyota downpipes.

Doing anything other than removing them from the old, and placing them in the new pipes has always led to bad results for me.

My general rule of thumb is if the 02 is working fine, let sleeping dogs lie. If you already have a code for it, and have troubleshooted to the point that it must be the o2, then it doesn't hurt to spray it with brake clean, or carb clean etc.... otherwise, it usually just leads to buying new o2's
 
Doing anything other than removing them from the old, and placing them in the new pipes has always led to bad results for me.

My general rule of thumb is if the 02 is working fine, let sleeping dogs lie. If you already have a code for it, and have troubleshooted to the point that it must be the o2, then it doesn't hurt to spray it with brake clean, or carb clean etc.... otherwise, it usually just leads to buying new o2's

Lesson learned if that is the case. I debated with myself before I sprayed the cleaner on them and I did it because I thought it made sense to
get that layer of carbon off of there since this would hopefully be the last time I would be seeing it for the foreseeable future.

I am going to try to test it, but I am already sort of resigned to the fact that my MUD OCD probably cost me $150. Oh well :hillbilly:
 

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