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OBD2 gives same code twice?

Hi all,

My Toyota Tacoma is reporting (via an affordable OBD-II scanner) the same P0420 code: First as a regular code, then as a pending code. What could that mean?


Well, technically it means “Catalyst System Efficiency Below Threshold (Bank 1)”.

Simplified, it means your cat converter ot downstream oxgen sensor(s) are getting weak.
How many miles you got on this puppy, anyway?. What year is it? Late model Toyota converters seem to be good for about 150,000 miles and then start throwing 420 codes.

Pending code is a problem that has been detected, but must re-occur a few times within a specific time frame to become a regular code that triggers the ‘Check Engine’ (CEL) light. This is a self check to prevent false ‘positive’ codes from sending you ‘screaming’ to the dealer. Once those codes get upgraded, they don’t necessarily cancel the pending code, so it can also hang around a bit.

I can’t tell you why it would show as both a pending and current code.

I can get a little more “technical” on what it means.

The O2 sensors “switch” - basically bouncing up and down in voltage based on the O2 content of the exhaust. The upstream (per-converter) O2 sensor will be very active because the exhaust is still “dirty.” Then the exhaust goes through the converter and gets cleaned up. So the downstream (post-converter) sensor should be way less active than the upstream sensor.

If the computer sees too much downstream sensor activity relative to the upstream sensor it sets this code. This might mean the converter isn’t doing its job (catalyst efficiency below threshold) - or it might mean that there is an issue with the sensor - or it might mean that something else, such as an exhaust leak is messing up the exhaust content.

The text mountainbike posted is the typical text that comes with that code. Its a terrible description and never should have been written that way.

Hi Folks,
This vehicle is a 2003 Toyota Tacoma with 130k miles, 2WD 2.4 liter.

By the way, I had the upstream bank 1 O2 sensor replaced about 8 months ago, for what it’s worth.

The upstream sensor would have nothing to do with the PO420. This is totally based on information fed to the ECU by the downstream sensor.