P040 code

JIM116

MEMBER
2000 TOYOTA TACOMA
2.4L
4 CYL
2WD
MANUAL
128,000 MILES

Burns no oil, regular tune ups, runs excellent. Gave inefficient catalytic converter code once on highway trip so I decided to change both O2 sensors since they were old. Now shows P040 code after ten to twenty miles of low speed driving. Installed two new Denso of correct numbers, no exhaust leaks, catalytic converter at inlet about 500 plus degrees.
Any ideas what to try next? Thanks.
(P.S. California emissions vehicle)

9 Replies

ASEMASTER6371

MECHANIC
52,797 POSTS

Good evening.

Can you please verify the code? You are missing a number at the end.

Roy

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Wednesday, July 25th, 2018 AT 3:06 PM

JIM116

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5 POSTS

Sorry about that, P0420, bank 1 low efficiency.

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Wednesday, July 25th, 2018 AT 3:12 PM

ASEMASTER6371

MECHANIC
52,797 POSTS

Okay, thanks.

You need a catalytic converter. There is no issue with the O2 sensors at all. The fact that the rear O2 voltage is reading the same as the front is the reason the code set. The rear should be steady and not going up and down like the front.

The precious metals are worn out internally and the catalytic converter needs to be replaced.

Roy

CIRCUIT DESCRIPTION
The ECM observes the wave form of the heated oxygen sensor located behind the catalyst to determine whether the catalyst is performance has deteriorated. If the catalyst is functioning normally, the wave form of the heated oxygen sensor located behind the catalyst switches back and forth between rich and lean much more slowly. When the wave form of the heated oxygen sensor located behind the catalyst alternates flatteringly between rich and lean, it indicates that catalyst performance has deteriorated.

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Wednesday, July 25th, 2018 AT 3:19 PM

JIM116

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5 POSTS

Thanks, I was hoping it was not that. One more question. With the old 02 sensors I went many weeks since the first P0420 code and never came back on. As soon as I put two new Denso O2’s in it started to show the code every day. Is that common? Are the new sensors just more sensitive or accurate in measuring the imbalance?

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Wednesday, July 25th, 2018 AT 3:31 PM

ASEMASTER6371

MECHANIC
52,797 POSTS

The new ones are just more sensitive.

It sounds like you cleared the code, the light went out and did not come back on until the PCM tested the converter and it failed its internal test three times in a row.

Clearing a code does nothing. The issue has to be addressed.

Roy