Do I need new catalytic converters or new oxygen sensors?

Question

I have collected freeze frame data from my truck. Does any of this tell what the problem for the code could be?
DTC for which Freeze Frame was Stored P0420
Fuel System 1 Status Open Loop - due to driving conditions
Fuel System 2 Status Open Loop - due to driving conditions
Calculated LOAD Value 2.75 %
Engine Coolant Temp 194.00 °F
Short Term Fuel Trim - Bank 1 -2.34 %
Long Term Fuel Trim - Bank 1 7.03 %
Short Term Fuel Trim - Bank 2 -2.34 %
Long Term Fuel Trim - Bank 2 4.69 %
Intake Manifold Absolute Pressure 10.07 inHg
Engine RPM 545.50 rpm
Vehicle Speed Sensor 0.00 mph
Air Flow Rate Mass Air Flow Sensor 0.82 lb/min
Absolute Throttle Position 0.00 %

Answer 1

You need to monitor the 02 sensors while the engines warmed up and running. Make sure they're switching back and forth. One may be sticking. Also have an exhaust back pressure test performed or monitor the vacuum readings to see if the cat(s) are suffering from internal meltdown or restriction. Most of the time it will be one of the two or both. .And yes it's true that the majority of the time the cat goes bad due to misfires etc...good luck and hope this helps.

Answer 2

This must be in your 1998 Silverado you mention in an earlier question. StPaulGuy is on the money for the most part, but I am wondering a bit about the long term fuel trim being positive and the short term trim being negative - especially exactly the same, like they are against a limit. No reason to suspect the O2 sensors; they are just the messengers. The mixture actually is messed up.



Those are still small numbers but taken in combination with being open loop I suspect something is forcing the mixture lean. P0420 is the result of the downstream O2 sensor on bank 1 seeing a high and fluctuating level of free oxygen in the exhaust leaving the bank 1 converter. With both banks in open loop and trying to lean out the mixture, that is producing the P0420. It is luck you don't have a P0430 as well.



Cleaning the MAF sensor and checking for vacuum leaks is a good start. Don't even worry about the converters until you can get both banks in closed loop when warm at idle. I am more than 90% certain the P0420 will go away when it goes closed loop.

Answer 3

First, knowing the year, make, model and engine would have helped. The computer is still in OPEN loop which means it's running off factory programming and not using live data. At 194 degrees engine temp, it should be in CLOSED loop and responding to input from the oxygen sensors. Next time take the freeze frame data while the vehicle is in closed loop and DRIVING. You did yours at idle.



The fuel trim data is within normal limits.



But I'd want to see oxygen sensor data from the upstream sensors. Don't replace anything yet. And keep this in mind, cat converters don't die on their own, they're killed by misfires, leaking fuel, oil, or coolant--by impact or by thermal shock.



Robert will tell you it's all because of the idle air control. No harm in cleaning it. But it won't fix your problem.

Answer 4

I am guessing this is a GM TRUCK?? WRONGO?? anyways forget the DETAILS! Begin with TWENTY DOLLARS spent at ANY OIL CHANGE CENTER YOU LIKE and have your THROTTLE BDFOya nd IDLE AIR CONTRO VALVE cleaned up to SHOWROMM clean! The IAC fien tuesne the amount of AIR enterign your engien and a CLOGGED UP ONE will RUIN READINGS and make you THINK you have BAD o2 sensors or a CLOGGED UP converter! NEVER REPLACE THE CONVERTER Unless it is ROTTING or LEAKING at the seams! Or got SQUASHED in a SERIOUS UNDER BODY crash!ONCE teh IAC is CLEANED UP the O2 senosrs can give a SIGNAL to it that wilL CUASE proepr AIR INTAKE! A dirty IAC wil CAUSE O2 senosr issues and eventually a CONVERTER FAILURE or at least a CLOGGING UP! They add EXTRA AIR based on those O2 signals! GOOD LUCK!!