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I recently purchased a 1998 Toyota Sienna. After driving it 800 miles on a road trip the code P0420 (catalytic system efficiency below threshold)came up along with P0115 (engine coolant temperature circuit). I am sure I need a new catalytic converter as the exhaust emissions are moderate. I am going to assume that the PO115 code which is telling me that I need to change put a circuit that reads my engine coolant was triggered by the P0420 code and perhaps a false code? My question is has anyone seen this if so please share your story. Also I know catalytic converter are not cheap however is there a way to get one less expensive? Thanks
 

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Over the years, there have been many P0420 problems on Generation-1 Siennas. Often, it's because of "laggy" old sensors before and after the converter. There are many threads on this site.

Before you buy a new cat, first, is your cat original? How about the sensors? Since, if you get a new cat, you'll probably replace the sensors anyway, you might think about replacing the sensors, and seeing what happens. Save yourself grief and only use Denso sensors. The ECM is set very strictly, so throws codes at minor variations.

My experience was that my before-cat sensors (A/F-ratio sensors in the 2002) went bad. I replaced, and was left with P0420. After replacing the post-cat oxygen sensor, no more codes, and the OEM cat worked fine for 235,000 miles.

How many miles on the van?

Search on here about the P0115. I never had to with this issue personally.

It's helpful to have a decent code reader that can read the emissions monitors and pending codes easily, if you're DIY-ing.
 
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