P0420 trouble code with a "Rich to Lean sensor threshold" error?

Question

My friend's car just threw a P0420 code, "Catalyst System Efficiency Below Threshold". We took a scan tool and checked the codes and Mode06 errors.

We see only the P0420 code, but also one failure in the Mode06 data:

"Rich to Lean sensor threshold - NOT OK
Max: 384
Current: 583"

Could this be an O2 sensor triggering the CEL? Really hoping it's not the catalytic converter.

Answer 1

Hunt down the sensor error first. P0420 is thrown by the ECU when the downstream O2 sensor reports excessive and fluctuating free oxygen in the output of the converter where the level should be low and relatively steady. If the code were renamed the more accurate "Excess oxygen in exhaust [bank 1]" few mechanics would replace the converter.



In my experience the code is caused by intake leaks, especially in one or more branches of the intake manifold; by exhaust leaks ahead of the catalytic converter, or by trouble with the mixture control (including dirty injectors). I have cleared a dozen P0420 codes for friends and family and have never replaced a converter to do it.

Answer 2

Most of the time, people tend to replace the CAT and the O2 sensor without knowing how the "Mystery" system works (Ie: Cat - O2 - PCM - etc)



Why didn't you mention the other DTC besides P0420 for the "Rich to Lean"? You have looked at mode 6 on a Subaru so what's the TID and CID (Really on 2003 called MID)? Why mentioned mode 6 on Subaru while you've no idea how useful about it?



Don't you know that from 2003 and newer models has what we've called CAN? On OBD II systems up to the introduction of Controller Area Network (CAN) electrical systems, there are two code identification tags for each line of data. The first is the Test Identification (TID) that indicates the monitor test, and the second is the Component Identification (CID) that identifies the sensor or other component that is being tested. After that come the actual test results. On CAN vehicles (those from 2003 to 2008 and newer), the TID is now called MID for Monitor Identification. It’s the same thing with a different name. Better yet, the MID test IDs have been standardized across different vehicle makes and models.



Depending on the capabilities and software in the scan tool or scanner software you are using, the hex code TID and CID values may be translated into plain English (or Spanish) followed by the hex code or decimal test results, the range of acceptable values for that particular test, the units of measure for that test (voltage, pressure, etc.), and a PASS or FAIL indication.



Mode 6 functionality is defined by the vehicle manufacturers and therefore is not the same from one make to another. This means you’ll have to check to see what’s reported for each make and model and determine its value. Mode 6 is also not supported by all scan tools, and may be unclearly defined by some tools that do support it.



Is Mode 6 data clear on every car? No. Will Mode 6 help every time? No. It’s worth the effort to check it out, though. Ask your scan tool manufacturer how to access Mode 6 data on its equipment. This data might save you some serious time and money on your next diagnosis. It’s just another tool in the diagnostic arsenal.



"Nuff" said: You'll need to know what the Oxygen sensors does: The front (Before CAT) and the rear (After CAT). This is the area that a majority DIY failed to understand as well as some of the people claimed that they've known "How to fix car" ..



A lazy O2 switching has triggered P0420/ P0430 code? None sense. The PCM/ ECM would give you plenty warning before hand. I'll accept that the P0420 has stored in the PCM as a result of a failed CAT due to a various reason that made the CAT failed to function and reports by the working O2 sensors as inputs to the PCM/ECM



Something has "Killed" the CAT.... a prolong misfire, a dripping injector or a coolant leak into any intake system..then make their way to the O2, kill it and then the CAT will be the next victim .. during this time, the O2 sensors if still working: Report the oxygen contain in the exhaust system as the inputs to the PCM.



So what are the readings the Subaru has? Hope you have some right ideas for your diagnostic ways. It could be the CAT or the O2 but what triggered the P0420?



Want the answer or a "Silver bullet"? Give me some more realistic readings, start with the freeze frame readings when the P0420 occur plus other DTC's and what you've checked/ testing..



Question or need to know about how the system works when P0420 triggered? [e-mail]



Otherwise: Don't forget to close the question. Thank you.

Answer 3

Year, make, model, engine size, mileage - please?



MILEAGE PLEASE?



Turbo?



Gotta love my happy followers, 3 thumbs down for wanting know some info in order to help someone. At least have the stones to leave a comment and say why.



88,000 miles doesn't seem like enough to justify replacing the cat under normal circumstances...unless it's a turbo and it gets beat on a fair bit. If the car is driven and maintained well FlagMichael is likely correct and your 420 code could be due to external forces. I'm thinking spark plugs, have they ever been changed? Deteriorated plugs can cause subtle misfires that introduce extra O2 into the exhaust stream, possibly a dirty MAF sensor, probably wouldn't hurt to run a bottle of good quality injector cleaner through with 1/4 tank of gas. After that if the code persists you can take the car to a shop and ask them to do a cat efficiency test where pre and post cat gases are measured to determine what's really going on before spending the big bucks.

Answer 4

P0420 Trouble Code

Answer 5

look at this site - http://www.p0420.com/ - for info to help you