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P420 error code after changing the catalytic converter


Topic starter

Hello everybody. I have a problem with my car that just destroys my nerves. 

I have a Suzuki Swift 1.2 16v 94hp from 2011 with 256k km. Recently I had the error code p420 for the catalytic converter efficiency. After changing the downstream o2 sensor, the values were jumping from 0.75v to 0.1v so that was clearly an issue with the CAT. 

I bought a new CAT. Not OEM, but OEM quality certified, it is full with material not just a little piece like other aftermarket ones have. 

I though it will also be a good idea to change the MAF sensor and also the MAP one because they were the original ones. Bought OEM bosch. 

After like 70-80 km, the error code came again on and I felt like I am gonna lose my shit. Checked again the values for the o2 sensors, but this time, the downstream one shows values between 0.65 and 0.8 which from what I read online is ok for a perfectly working CAT. 

With the error deleted, the fuel trims are lower, between -1 to -4 and the car runs great. After the error is back on, the trims go to like -10. Still withing parameters but of course it is in some sort of limp mode and it does not have so great throttle response. 

I have recorded some live data with the error on and the error off. I do not know what to do honestly. The main O2 sensor, the wideband A/F one, is not new but around 100k km. At some point I ran some catalytic converter cleaning through the engine. The one that is sprayed in the intake. Is it possible that it damaged the main o2 sensor? The values don't look bad for it. 

Also when the error is off, the instant fuel consumption is lower than with the old cat, so even more indication that the car is running great.

I have no clue what to do next to get rid of the error code.     - This is the live data with the error code OFF

   - This is the live data with the error code ON


Please help Crying  

This topic was modified 3 weeks ago by mutulica2
7 Answers

First off just because your O2 numbers are doing that just doesn't mean your cat is bad. Exhaust leak, faulty sensor, wiring, harness ect all can cause that.  Since you delete codes and that helps you probably have a failing or lazy sensor or possible damaged. Since you used OEM that rules out a few things. Before anything check all wiring and connections. I've seen shops forget to plug sensors in or damage wires many times. 

You can see in the videos that I have information from both o2 sensors, so wiring is fine, they are both plugged in, checked for corroded connection, all fine. As for exhaust leaks, I checked yesterday, the new cat fitted perfectly and there seems no to be any..


I was finally able to watch both of them and the only thing I see off is your timing. It's really jumping around a lot.  The numbers sound stay near the same or have a change of .5 to 1 anymore That usually happens but not always from a bad tensioner or stretched belt or chain.  You can try pressure testing the exhaust to make sure new cat is working properly and had proper flow for your car.  Also remember just because parts are new doesn't mean they are good. At least 5% of sensors or parts I've replaced had a defect or issue.  You said issues happened after down stream was replaced. Try putting in old sensor.

Topic starter

Coming with some updates. So by now I did this. Changed both o2 sensors with OEM parts, new catalytic converter, new MAF sensor, new MAP sensor. Replaced all the exhaust gaskets and checked the pipes for leaks, nothing detected. I still have the p0420 error code although I honestly cannot find anything wrong at this point with the car. The car is running great, fuel consumption went down, no rough idle, no nothing. I attached now 2 videos with live data, one at IDLE and one at around 2k rpm. In the 2k rpm video you can see the o2s2 showing 0.1v but that was because I was struggling to keep the rpm and it was decelerating a bit. After it got steady, it went away. What else should I check? Could it be something related to the ECU?


- @2000 rpm

Topic starter

earlier today, noticed something strange. With the AC on, the o2 downstream sensor started to randomly switch from 0.7 to 0.1, 0.2. When turning the AC off, it stays steady around 0.7. I start to think that the car is possesed by some o2 sensor demon. Tomorrow I am going to clean all the grounds I can find in the engine bay, maybe there are some corroded or something and giving strange signals in the electrical system. While this switching appears, the engine runs perfectly fine, no hesitation, no rough idle, no nothing. 


 . I watched all the videos few  more times and there is a couple take aways. Your battery voltage isnt consistent. I noticed your voltage was 12.1 volts all the way to 14 at one point. The voltage should be pretty consistent when running and turned off. Like my Mercedes when car is off voltage is 12.65 once I start it to goes straight to 13.9 and it stays there. It may go up and down by 0.1 to 0.5 but steady for the most part. You could have a bad alternator or voltage regulator. I'm not sure if your cars is one unit. I'll have to pull that up tomorrow. Another possibility is a worn or damaged belt causing slipping or a loose connection or corrosion. 

My main concern is your timing it shouldn't be doing that at all. Those numbers should stay nearly the same with very very movement. When it does fluctuate it should be no more then 1.5 maybe 2 if car is older and it should happen slowly. Main cause is worn tensioner, stretched chain or belt or you jumped a tooth.. 

There is a slim possibility the  crank position sensor or cam position sensor are at fault. If you do have a voltage issue that can cause some systems and sensors to not work properly and can be what causing timing to act like that.

Personally I feel it's something with timing.

Im almost certain the down stream issues are a side effect of something else that's going on . We just have to narrow it down and well figure it out. Tomorrow I'll look more into your car and see what I can find out 

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The voltage is doing that because it has smart charging, when it doesn't need it, it cuts it down. It's a feature for low emissions. Here in Europe, these things are quite looked into. The car is with the Euro5 norm. If I turn on my lights, the voltage gets steady to 13.6 and when I am using the engine break, it's at 14.6, otherwise, it's doing it's own things. I thought it was an issue too but when I looked into it I saw that it's normal baheviour. Regarding the timing, the car has VVT. It has a timing chain. The chain is not knocking at first start up at low temps, not even at -20 Celsius (-29 fah).

Don't know if it's related but when it shows low voltage on the secondary o2, the timing is somewhere at 0.5, 1.0 degree.

Topic starter

Coming with some updates. 

Cleaned the fuel injectors today, the spray perfectly, creating a nice mist. 

I am going to drive a few days, maybe it changes but I don't understand why I have -10, -15 fuel trims at some point and then it stays casually at -3, -4 for a while, and then again goes up. 

Sometimes I feel the car really nice, with low instant consumption on the dash, other times the instant cons. is higher, with the same driving conditions.

Next week, I am going to the dealer to check the software in the ECU. Maybe something is corrupt, maybe there is an update or so.