Brian1888

Pre Cat Lambda sensor.

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I have Toyota Aygo 2010..EML comes on and code states P0420 which i think is the pre cat sensor.

The part for this vary hugely in price. Does anyone have any advice on what part to install without being too expensive.? 

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P0420 Is not specific to an individual item/fault it is indicating the Catalyst is below efficiency:

This vehicle has two heated oxygen sensors. One is located in front of the Three-Way Catalyst Converter (TWC), and the other is mounted behind the TWC. Each signal output of the sensors is converted into a waveform inside the ECM. The front heated oxygen sensor is used to monitor the air-fuel ratio by the ECM, and whose signal prompts the ECM to perform air-fuel ratio feedback control. As a result, the air-fuel ratio is balanced, and the waveform of the front heated oxygen sensor regularly oscillates between rich and lean. To determine whether or not the TWC performance has deteriorated, the ECM compares the waveforms of the front and rear heated oxygen sensors. While the TWC is functioning normally, the waveform of the rear heated oxygen sensor fluctuates between rich and lean more slowly than the front heated oxygen sensor's waveform. When the rear heated oxygen sensor's waveform frequently fluctuates, it indicates that the TWC performance deteriorates.

Possible Trouble Areas:

Gas leakage from exhaust system

Heated oxygen sensor (sensor 1)

Heated oxygen sensor (sensor 2)

TWC

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In short, it could be either your 1st or 2nd sensor. Or both.

Cheap sensors usually fail far earlier than genuine OE ones. Yes the OE is expensive. Or you might be looking to sell the car next year. Yer pays yer money.

Edit: or is could be the cat itself...

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Hi

We had this exact problem only recent, however we replaced the cat with a brand new genuine toyota part, reason for this we got very lucky on ebay, winning bid £35.00, you read correct, and we replaced both pre n post sensor with genuine Bosch parts, we havent seen the eml light on since.

Do you know any mechanics who’d be willing to order the parts on their account saving you some money maybe?

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I would try personally changing the sensors, with something from the scrap yard if im lucky to find it.

Sent from my VTR-L09 using Tapatalk

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Thank you. I appreciate all advice. I am going to try a Pre cat Bosch sensor  1st.

I think this looks easy enough to fit.?? 

Do i fit new sensor then erase code or erase code first then fit part?? 

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On 4/23/2019 at 9:34 AM, Brian1888 said:

Thank you. I appreciate all advice. I am going to try a Pre cat Bosch sensor  1st.

I think this looks easy enough to fit.?? 

Do i fit new sensor then erase code or erase code first then fit part?? 

When I had the same issue, I also changed my catalytic converter but I reasoned that I should change the 2nd sensor instead of the first. I think the reason was because the 2nd sensor was cheaper than the 1st one? But it solved my issue. As has been said, the code just points to an issue in that area.

As for ease of fit, it's not too bad. It might benefit from a bit of spraying of Plusgas before removal for a few days before hand (just to make it easier to unscrew), but it's simply a case of unscrewing it and plugging out, and the reverse for fitting.

With the code, it doesn't matter. You can clear the code after fitting it or before hand.

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