2004 Tacoma, 5VZ-FE, 190K miles.
p0420 code showed up last week. Changed air filter (it was close to that time, so did it anyway), cleaned MAF. Cleared code, showed back up within a half tank.
From some research sounds like it could have been the MAF, one of the two O2 sensors, or the cat.
My question is, what is the next logical step to track this down? I don't want to spend the money or time replacing a bunch of stuff I don't need to.
I've read of someone who was going to test their O2 sensors. How does one go about that?
Any and all help is appreciated.

 

Just remember that code includes every thing in that circuit .

Here is where having a scanner to read real time data. is nice.

Checking an O2 sensor with a meter is a real pain real easy to burn your self and the meter leads because you need to read the voltage while the engine is running.

Back probing with long meter probes at the plug.

The voltage can be different between the years and I am not sure of the 2004

Remember this code is just that the ecm does not see enough difference between the upstream and downstream O2 sensors

Lots of people just change the down stream sensor and hope.

As you found out if the problem is not fixed when the test cycles the Mil trips again

 

I'm one of the ones that gambled on the rear O2 and won. I knew it was original at 120k and the front o2 had been done at 90k, so it was a pretty good bet. At 170k now. It comes on once in a while if I'm driving slowly for a long time, but goes away after reset.

I also got one at about 150k when the gasket on the upstream end of the downstream cat rusted out. $6 20-minute fix. Check carefully for exhausts leaks from the last cat all the way back to the manifold.

Otherewise, you really need a scanner. (I have one now).

 

Drove my truck for a couple years with a po420 pending. It rarely ever tripped the MIL. The rear 02 sensor was just zip tied up under the truck because I never bothered to hook it up. So your truck's catalyst system is below effeciency. How's your gas mileage and power?

 

Thanks for the replies, just getting a chance to get back to this today. I'll be combing through maintenance records today to see if my wife had any work done in regards to the exhaust system before I started maintenance on it.
@ vasinvictor, yes both power & mileage are down.
@ wyoming9, you talk of a scanner to get real time data. Guessing you mean one more specific than the general code reader used by napa to pull & clear the p0420. Any recommendations for one?
Again, I'd like to track down the problem & fix it instead of just replacing everything that could possibly be wrong.

 

You didn't ask me, but the cheapest entry live scanner is an Android smartphone, the Torque Pro app, and a ELM327 Bluetooth OBDII adapter off e-Bay or Amazon.

Live scanners give you access to a whole world of real-time data about your engine, including temp., RPM's, Vacuum/Boost, sensor readouts, etc. Everything the ECU uses to control your engine and set codes.

 

Ordered, thanks Dirk!
After combing through records it looks like both O2 sensors & Cat is all original. Had a recommendation to replace the front O2 sensor in the past due to another code (p0171). If I don't find any exhaust leaks I'll start there.

 

At 190k, neither of those sensors owe you anything. Even if you end up needing a new cat, you'd want new sensors to go with it. Any shop would insist on it. So change them first, and if they fix your problem, you're golden. If they don't, you really haven't lost anything. Assuming it wasn't an exhaust leak...

 

Checked under the truck for exhaust leak. Only thing I found was a small hole in the muffler, down stream of the 2nd O2 sensor. Could this be the issue?
Comments on this thread, as well as other reading, has lead me to believe that the code is based on info derived from the upstream & downstream O2 sensors. Therefore anything below this wouldn't cause a code trip. Wrong conclusion on my part?

 

You are correct. A leak downstream of both o2 sensors will have zero impact on those codes.

 

Hopefully, you got it figured out. Sensors have a replacement interval. I guessed mine were due, but also got that dreaded code. So I replaced them. Code came back after about a year and a half. Found an exhaust leak at the converter flanges (the one between front and rear if you have Cali-type) and fixed it. Still code wouldn't clear.

So I got an INNOVA scan tool. Since my sensors were new, I read about bad cats and correlating sensor voltages. Short and sweet, bad cats would throw your rear sensor readings all over the place no matter how constant the vehicle speed. After I tore that converter down and put in an aftermarket, the rear sensor voltage was much more steadier (still fluctuates within spec) at constant speeds/idle.

Sensors and exhaust leaks are culprits. If caught early enough, can save the cat. But for me, that code came back several months after I replaced both sensors. Leaks are a pain to find. I sprayed the suspected area with soapy water while the engine ran.

If it really is just the cat, driveability and performance may not even be an issue. A bad cat will only affect how clean your runner can reduce emissions.