I have a 2000 Toyota Tacoma with 250k miles on it that runs like a champ. But, after some repairs, the catalytic converter monitor (or catalyst monitor) has never set. Every mechanic has told me initially to "do a drive cycle" on it (or even "Google 'OBD-2 drive cycle'"), but also that 50 miles of driving should cover the requirements. Yet, I have driven for months and probably 10000 miles and yet the monitor still shows as incomplete.

How can I tell which part I need to replace to fix this issue?

  • I have already replaced the O2 sensors and the thermostat
  • A forum discussion has suggested the thermostat or the Throttle Position Sensor (TPS)
    • The TPS would manifest itself in other issues, no?
    • I don't know if they mean thermostat or temperature sensor
      • My OBD-2 reader reports the temperature at 180º-200º F during normal driving conditions
  • Would replacement of the catalytic converter even affect the monitor? I don't want to spend $300+ for a red herring
    • The Toyota dealership told me that my catalytic converter doesn't have the right voltage to pass or the right voltage to fail, so that I would have to replace my catalytic converter. They then quoted me an outrageous sum to do so. I still haven't decided if I believe them in full.

2 Answers 2


The monitors are usually run easily on this engine setup. The following pre-conditions must be met for running the monitor: Coolant temp above 176 deg F, Intake air temp above 14 deg F. No trouble codes set or pending.

Replacing the catalyst will not affect the monitor. The monitor test runs whenever the right conditions are met regardless of catalyst quality. Verify the above conditions are met. Then drive the drive cycle.

If the engine coolant thermostat is stuck open it will be too cold and never run the test. If the temp sensor has failed the PCM was will think the engine is too cold even if it is not. If any codes are set or pending it will not run. Pending codes will not turn on the check engine light.

A simple OBD-II scanner is the best tool for these tests.


I disagree with your Pending Code statement. The monitors run with pending code. After two consecutive pending code you will end up with permanent code.

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