200 Tacoma 4x4 2.7 4 cyl error codes

This code means just what it says. The engine coolant is not getting warm enough for closed loop operation. This usually means the thermostat is stuck open. If the thermostat was stuck open you would have little to no heat and the engine temp guage would stay at the cold mark or take a long time to reach normal temp ( middle of guage )'

Then why couldn't he write it down on a piece of paper ???? From the info I found a P0135 code indicates O2 sensor heater cicuit malfunction - bank 1 sensor 1 This could be as simple as on O2 sensor gone bad ( common) or it could be a wiring problem ( not so common ).

Coolant temp sensor would be the same in any car that used one. It tells the computer what temperature the engine is at.

probably the same here but what about the 125 code. He

Your truck will continue to run like crap and your fuel mileage will drop.

It could be your Haynes manual is wrong. All the info I found for the P0125 code says it is the coolant sensor. That's easy to check by watching the engine temp gauge.

For the P0135, O2 sensors are a wear item that need to be replaced at high mileage. If it were mine I would just put an O2 sensor in it.

I would expect that the core problem is that the coolant temperature is too low. A stuck open thermostat is the first thing to suspect. You ought to be seeing the temperature gauge reading too low and you're probably getting very little heat out of the heater especially at highway speed. If the coolant doesn't get to normal temperature, the fuel system cannot go into closed loop and the oxygen sensor output is probably too low and not changing. The computer logic can interpret a low, constant oxygen sensor output as a heater circuit malfunction and thus set the P0135 code. Get the coolant temperature problem fixed and the O2 sensor code will likely disappear too. Good luck. Al

This is correct.

If the coolant doesn't get to normal temperature,

If the computer doesn't go into closed loop it doesn't use the O2 sensor output.

The computer logic can

Wrong. The computer will not interpret a low O2 sensor output as an O2 heater circuit failure. The heater circuit is a seperate circuit of the O2 sensor and has nothing to do with O2 sensor output voltage.

Get the coolant temperature

The coolant temp problem will have nothing to do with an O2 sensor heater failure.

Heater works perfect, replacing the CTS tomorrow, will see what happens then.

Try this,

Assuming you have described the errors accurately -- I haven't looked them up -- they are related to one another.