Wilkems Posted January 6, 2016 Share Posted January 6, 2016 I'm trying to diagnose an ongoing p0420 code. I hooked up an ODBII sensor and graphed the voltage of the o2 Sensors. The one after the pipe holds constant between .5-.8 volts which I understand is normal. After the car came up to temperature, the up pipe o2 (or fuel or whatever) sensor never posted any voltage. What I've seen is that it's suppose to bounce up and down once or twice a second or so. Instead it's nothing. My mileage has dropped 3-4 mpg and I'm hoping this is all related. My hope is that since the front sensor isn't working, the fuel mixture is rich which could be junking up the cat, causing the rear sensor to throw a code. Any ideas or suggestions would be much appreciated! Link to comment Share on other sites More sharing options...
compsurge Posted January 6, 2016 Share Posted January 6, 2016 What year car? Do you have a turbo (up pipe is for turbo typically)? Have you tried measuring the resistance on the sensor? Search here for "vacation pix" to help get the right value and troubleshooting methodology. If your sensor measures correctly, try doing a continuity check between the harnesses. You may want to unplug and reseat the connectors as well. Check to make sure the crimp terminal pins are seated properly. Link to comment Share on other sites More sharing options...
NORULZleggy Posted January 6, 2016 Share Posted January 6, 2016 Or you could hit up Rockauto and by a new one . I bought 2 for my cars and they were on sale for 48 each. You can also get some 02 sensor cleaner and try that. Link to comment Share on other sites More sharing options...
Wilkems Posted January 6, 2016 Author Share Posted January 6, 2016 Thanks guys. It's a 2005 2.5i Limited. Up pipe is in reference to just being closer to the engine. Sorry for the confusion. I have one already coming in. I'll try to unplug and reset the harness.I had cv boot blow a while ago that sprayed gunk everywhere. I'm wondering if it finally killed it over time or something. Link to comment Share on other sites More sharing options...
whitetiger Posted January 6, 2016 Share Posted January 6, 2016 P0420 is for the rear sensor generally. Link to comment Share on other sites More sharing options...
Wilkems Posted January 6, 2016 Author Share Posted January 6, 2016 P0420 is for the rear sensor generally. I know that's usually the case. But my rear sensor is properly working which usually means it's the Cat. I'm wondering if the front sensor not working could be creating a lean mixture which is gunking up the Cat and causing the rear sensor to throw the code. Link to comment Share on other sites More sharing options...
whitetiger Posted January 6, 2016 Share Posted January 6, 2016 you are over thinking the issue. replace the rear sensor. just cause you read a constant voltage int he rear, does not mean its responding properly. somthing is saying that the cat is not good. only the rear sensor does this. so either the sensor is bad, or the cat is bad. if the cat was bad, and the rear sensor good, the reading would be either on the low or high end of the range, or it would fluctuate too fast for the ecu to think it was good. replace the sensor. Link to comment Share on other sites More sharing options...
Wilkems Posted January 6, 2016 Author Share Posted January 6, 2016 you are over thinking the issue. replace the rear sensor. just cause you read a constant voltage int he rear, does not mean its responding properly. somthing is saying that the cat is not good. only the rear sensor does this. so either the sensor is bad, or the cat is bad. if the cat was bad, and the rear sensor good, the reading would be either on the low or high end of the range, or it would fluctuate too fast for the ecu to think it was good. replace the sensor. Thanks. You're probably right. I'm probably being hopeful. Since I already have the front coming in I'll do that anyway since it's not working and see what happens, then follow up with the back. Hopefully it will at least help with the fuel issue. It started before it got cold so I don't think it's solely the dreaded 5th Gen winter mileage drop. Link to comment Share on other sites More sharing options...
gwicks Posted January 29, 2016 Share Posted January 29, 2016 I have the same code and the same issue with finding my front o2 sensor voltage. Pretty sure it's because the front o2 sensor is actually an AFR (air fuel ratio) wideband sensor. When i was diagnosing mine I logged the o2 1 x2 and the AFR(m)....i got pretty similar readings to you...rear steady at about 0.8v, front jumping around, up to 5.5v. Still just clearing the code at the moment, will replace both sensors with the DENSO oem ones from amazon, $50 for rear, $90 for front. Should be doing in a few weeks, will let you know what happens. Link to comment Share on other sites More sharing options...
gwicks Posted January 29, 2016 Share Posted January 29, 2016 I know that's usually the case. But my rear sensor is properly working which usually means it's the Cat. I'm wondering if the front sensor not working could be creating a lean mixture which is gunking up the Cat and causing the rear sensor to throw the code. If it was the cat, my understanding is it would be jumping around rapidly, by holding steady it should mean the cat is ok. I also bought a laser temp gauge and checked my cat once it was up to temp. Rear was 100 hotter than front of cat, which also should mean the cat is ok. As well as replacing the 2 o2 sensors i'm going to replace the MAF, as my car has 115k on it and have heard they are pretty shagged by then. Link to comment Share on other sites More sharing options...
NORULZleggy Posted January 29, 2016 Share Posted January 29, 2016 If it was the cat, my understanding is it would be jumping around rapidly, by holding steady it should mean the cat is ok. I also bought a laser temp gauge and checked my cat once it was up to temp. Rear was 100 hotter than front of cat, which also should mean the cat is ok. As well as replacing the 2 o2 sensors i'm going to replace the MAF, as my car has 115k on it and have heard they are pretty shagged by then. wait, I would not touch the MAF, I have 150K on my legacy, and 173K on my outback. I use CRC mass air flow cleaner 3 times a year and that keeps them nice and clean. just get a can spray it good, let it dry and install it. no need for a new MAF. Link to comment Share on other sites More sharing options...
fullmonte77 Posted January 30, 2016 Share Posted January 30, 2016 Had the same issues on my 07. I replaced the front sensor and it fixed it for a few hundred miles. I was throwing a code every 25 miles or so. I replaced the rear sensor and it worked for a while and then the codes came back. I did some research and found that winter gas and cold temps could have been the culprit. Well low and behold after the days started getting warmer the codes would take longer and longer to come back until they stopped completely. It is now winter again and no issues(knock on wood). Sent from my iPhone using Tapatalk Link to comment Share on other sites More sharing options...
Wilkems Posted February 22, 2016 Author Share Posted February 22, 2016 Just wanted to update everyone on what happened. Turned out it was the front O2 (fuel air ratio) sensor. Ordered one for $42 off ebay and at first it didn't work. Turns out it feels like it clicks in but you really have to get in there and squeeze and it will pop all the way into the harness. I ride this cheap sensor out until it goes and then maybe I'll upgrade to the Denso. But it works for now so why change it. Once I got it in all lights have gone away. No more P0420 code and I didn't touch the back sensor. Also my gas mileage has gone up about 5 mpg! Thanks everyone for contributing. I'll keep everyone posted if something changes. Link to comment Share on other sites More sharing options...
NORULZleggy Posted February 23, 2016 Share Posted February 23, 2016 Great Update, I have 2 Denso front 02 sensor's sitting in my garage that are New. I think once my outback hits 200K and my legacy hits 150K I will change them, so far they are fine, but if they fail sooner I will change them. The MPH is fine so far. Link to comment Share on other sites More sharing options...
dgoodhue Posted February 23, 2016 Share Posted February 23, 2016 I'm trying to diagnose an ongoing p0420 code. I hooked up an ODBII sensor and graphed the voltage of the o2 Sensors. The one after the pipe holds constant between .5-.8 volts which I understand is normal. My hope is that since the front sensor isn't working, the fuel mixture is rich which could be junking up the cat, causing the rear sensor to throw a code. The rear O2 sensor should be reading 0.1 to 0.5V when everything is operating properly. What is it now reading on your OBII reader? Link to comment Share on other sites More sharing options...
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