We have a virtually bone stock T4R with around 200k on it. It's my wife's truck, and it's a street queen. My problem is my memory has gotten fuzzy and I can't remember a lot of details right now.
1.) How many cats does it come with stock?
2.) If it only has 1, will it continue to throw P0135 and P0141 codes?

I have replaced both O2 sensors (multiple times using OEM and cheap ones), and erased the codes with a scanner. The codes keep coming back when I unplug the scanner. If I were to leave it plugged in, the codes stay deleted.

Again, this is a mostly stock rig. I don't remember if it had a 2nd cat or not, but it only has one at this time. It feels sluggish, and just not right.

 

On cold start the ECU sends current to the O2 Sensors to warm them up fast so they function (maybe you knew this). The ECU also checks for open circuit or short. Its very odd that that both failed unless its a grounding issue. Check your engine block ground, especially the one near the Diag port thingy (on 5VZE).My wiring diagram (tacoma) says that is gnd for O2 sensors (EB).

If nothing wrong with gnds then there is a procedure in the FSM for diagnosing this but you need a multimeter.

 

I have a multimeter, but no FSM.

 

Here's the circuit, you can see Ht1 and HT2 go through the O2 sensor and then grounded at EB which means engine block. There is only one engine block ground, on the Tacoma its right next to the diag port on the intake, driver side. I assume 4R 5VZE is the same. Physically check that ground point - a lot of times when work is done on the truck people forget to attach that or its loose and I have heard that will cause a PO134.

Really difficult to believe its not a ground issue since the O2 sensors have independent heating circuits and the chance of a wiring problem on both is small unless the harness touched the exhaust or got mangled somehow but I assume you would have noticed this when changing the sensors. It looks like the wires go directly to the ground point, I believe its that wire harness that goes over the top of the transmission and near the driver side exhaust manifold and I think sort of turns back to the fire wall and joins the engine harness. I have also cringed a bit at the wiring harness so close to the exhaust manifold but it does seem to have some heat shielding on it.

I think the leaving the scanner on and codes do not appear is a red herring but not sure.

 

Also just saw something where the scan gauge can make the ground (when people say scan gauge not sure if they mean OBDII reader or the Toyota scan gauge) thus while plugged in no codes appear so it may be real after all. Again check all grounds on the engine, I would physically remove and clean. I actually saw that the EB ground also grounds the igniter so if ground completely off the engine will not start but poor ground also a problem because the heater is a very large current, I believe 7A and the ECU has a hair trigger on this value.

Checking signals at the ECU and various other hard to get places is very difficult, luckily I have never had to do it on my 98 Tacoma (yet). But I have done it on other cars and once I spent several hours tearing my dash part just to finally find out that I had not completely closed a connector in the engine bay. Also on these old cars its often very difficult to separate the conns and they can be brittle so it can snowball. I have found WD40 helps although it then makes the conn hard to grip.

It doesn't sound like you had recent work done on the vehicle but if so you will want to review that work before tearing into things and clean grounds since that is low hanging fruit. There are 2 additional ground straps on the 5VZE, one on the pass cyl head to fire wall and another big one near the oil filter that is hard to get to. These strap the engine to the body so are not as critical as the EB ground but are still necessary.

How in the world are you able to maintain 3 old Toyotas with no service manual - do you do maintenance yourself or just have very few problems?

 

Here is the test procedure from the FSM for checking voltage at the ECM and the O2 sensor.