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OBD2 Error code P0420 - Catalyst System Efficiency Below Threshold possible CHEAP FIX for any car.

There is data here that applies to a 2001 to 2003 Prius but the basic concept is applicable to any car that generates the ODB II trouble code of P0420 - Catalyst System Efficiency Below Threshold

This normal solution for this error is to change the catalytic converter however on a 2001 Prius to change that you have to change two catalytic converters and a carbon trap. Price just for parts was around $1,100. plus labor. (more now) At least that is the dealer solution and the only one available in California. In most other states, you can buy the front catalytic converter and have it installed separately.

This code is generated by the engine control computer in the car comparing the voltage outputs of two oxygen sensors in the exhaust pipe. One is before the first catalytic converter and the other after the first converter. The upstream sensor, before the catalytic converter, is also used by the engine computer to generate an optimal air/fuel ratio. So when it is not working right your mileage can suffer.

I was suspicious of the code because it would take 60 miles of driving for the error to occur and how bad could the catalytic converter be if the error only occurred after 60 miles of driving.

My mileage had been decreasing so I change both oxygen sensors. The mileage went from 40MPG to 47MPG after changing the upstream sensor and amazingly the error went away and there is no check engine light after changing the downstream sensor! Dec 2012 - It is now 9 months later and the error has not returned.

However, the problem came back and I eventually junked the car rather than pay the dealer price for a catalytic converter. However I have since learned that catalytic converter problems are largely caused by the engines burning oil. My Prius was burning oil and I found out how to solve this on my 2007 Prius that started to have the same problem. I have a write-up specifically for the engine in Prius 2001 to 2009 Prius C, & Echo. I also cover the overall procedure which applies to any gasoline engine.

Total cost in parts about $130 for both sensors (Internet shopping - you can do it for about 50% less with universal sensors but then you have wiring headaches) plus a lot of labor as the upstream sensor is hard to get to and was frozen into the exhaust manifold. I used the DENSO oxygen sensor because they were what was originally in the car and did not want to introduce any changes that might not clear the error code.

To replace the upstream oxygen sensor in a Prius you need to remove the wiper assembly which is time consuming but not particularly hard to do.

The upstream oxygen sensor is inserted in the top of the exhaust manifold just in front of the fire wall. There is very little space to get at it and it is often seized in the exhaust manifold. If it is seized and you are using the correct wrench or socket and some penetrating oil and can not loosen it do not waste a lot of time trying. (I spent hours and all I accomplished was a rounded sensor!) Jack the car up and remove the manifold cut off the sensor just above the manifold. A small cylinder should fall out of the sensor making it easy to line up the drill but use a drill press and a vice to hold the manifold still. Use bigger and bigger drill bits till most of the sensor is out and then re-tap. The exhaust gaskets in a Prius are very high quality and I have reused them.

For 2001 Prius if the VIN is lower than JT2BK1#U#10032180 the Prius might need a new Engine Control Module if the part number is 89661–47030. This is based on a Toyota Technical Service Bulletin EG017-03. My car has a newer VIN and did not have this problem.

The Denso Oxygen Sensors for a 2001 to 2003 (What Toyota used) are:
  • Upstream: 234-4624 (The one in the exhaust manifold)
  • Downstream: 234-4623 (After the first catalytic converter)
For a 2004-2009 Prius the DENSO sensors are:
  • Upstream: 234-9056 (In the exhaust manifold - Toyota used a better sensor to improve the fuel air mixture and the gas mileage.)
  • Downstream: 234-4623 (After the first catalytic converter)

Hybrid partsHybrid partsAs an owner of a Prius I have found some parts that you might want.(see below)

I have tested them on the five Prius cars I have owned. We have been selling 12 volt batteries for hybrids since 2005. The Optima deep discharge batteries work great on a hybrid. (These batteries have thicker lead plates to allow more discharges instead of lot of cranking amps. You do not need cranking amps on a hybrid because the high voltage battery cranks the engine.)

I have also been testing the Intimidator Prius Batteryand found it still had over 70% of capacity despite the fact that is was deeply discharged on several ocassions due to the car not being driven.

"A short time ago I sent an email requesting the USPS tracking number. I am now pleased to inform you that this information is no longer necessary, in light of the fact that USPS has already delivered my Prius battery. In my opinion this is nothing short of miraculous! Kudos to you and your shipper, the U.S. Postal Service!

The battery was very thoughtfully protected within the Priority Mail box, by the way. Thanks for your expert assistance!

Sincerely, J. H., Clarksburg, MD

P.S. I ordered this same battery about a week ago, from Amazon, whose Marketplace at that time was defaulting to a competitor of yours. Four days later, I received word from them that the battery was not actually in stock at their warehouse, but would arrive sometime within the next two weeks. When I pointed out that their online inventory through Amazon Marketplace had indicated that it was in stock at the time of my order, and that it seemed like they could have informed me otherwise in something less than four days, they got quite a bit "huffy" about it (putting it mildly). Anyway, more to the point, in less time than it took them to tell me that their own inventory control was deficient, you actually put a brand new battery in my hands, and shipped it all the way across the nation to get it here. I won't forget it!

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