P0420
no FSM> and not looking up P0420?
no engine stated, so ill get it wrong too..?????????????
420 is cat monitor failure. "efficiency"
the most hard of all.
ill make a list from hard and expensive .
1: bad sensor 2 rear. you failed to do both, that was WRONG.
2: bad cat. (i can weld on any cat for $99 bucks.) i do it last.. dead last.
3: engine running real bad. BUY a scan tool, now.
so the o2 is new that leaves #2 or #3.
i never shot gun cats. but shops do, its not their cash, huh?
i connect a real scan tool (not one that misses factory DTC codes)
then look at pendings for P042o , see funny sensor readings there, bingo.
i check every PID. for all sensors to be sure they READ right.
ECT AT 180F+ AND HOLDING? HOT.>?,
is the engine holding closed loop status at idle, no fix that NOW.
is LTFT (long term fuel trim ) near 0 or at - 40 or + 40% (fix that next)
if trim is good, the engine.
If the front 02 is new, the AFR will be correct and the the front 02 will swing fast and the trim will be near 0 , -4 is near best.
tad rich , natural.
once the engine is running Stoich at light flat cruise. (close loop) the
cat will heal its self (bake dry) and stop throwing 420. ( a hope deal)
if not the cat is bad.
ive cured some of the most hard 420s,.
on some ratty engines, a new cat can in fact hide it , until it cooks to death and next year 420 are back.
id never do that because gas is expensive. fix engine first.
SOURCE: Check Engine Light - O2 Sensor - bank 1 running lean
you should have three oxygen sensors on your car, one for each bank of three cylinders; should be towards the Y pipe on each exhaust manifold. Additionally you should have one on the exhaust pipe before the cadilitic converter.
SOURCE: P0420 and check engine light
A p0420 code is set when the computer determines that the catalytic converter isn't doing it's job. In almost all instances that code can't set as a result of a bad o2 sensor so they probably didn't have to be replaced. The catalytic converter will need to be replaced, prefferably by an oem part. This can be pretty expensive and should be considered before purchasing the vehicle.
I have replaced a few bad converters on late model subarus in the past few years.
SOURCE: 1998 Saturn SL2 engine light
Possible Causes
The following are malfunctions that could cause a rich condition:
^ Fuel pressure too high. Perform fuel pressure test.
^ Leaking fuel injector(s). - Perform pressure leak down test.
^ Vacuum leak around the MAP sensor grommet.
^ Oxygen sensor contamination.
^ Restricted exhaust or air intake.
^ Fuel Vapor Canister saturated with fuel.
IMPORTANT: If the front oxygen reading voltage is above 600 mV and the rear HO2S is above 600 mV all the time, the O2 sensors are probably not
at fault.
SOURCE: how do you change the downstream oxygen sensor in
Downstream O2 sensor is the O2 sensor after the Catalytic converter. Jack the car off the ground so you can get under it, disconnect the wire leads and with a close end wrench. Snake the wire leads through and wrench and place the wrench one the O2 sensor and with a hammer, tap the wrench to break lose the O2 sensor from the exhaust pipe. Once the O2 sensor has loosen up, just remove and install the new O2 sensor. Reconnect the wire leads, start the car and listen for any exhaust leaks. Once no leak has been determined, Place car back on the ground and go for test drive. Check engine light should go off after three start up and car has reached it's operating temperature and DTC codes will be erased after 100start cycle. Good luck and be safe.
SOURCE: 2000 Subaru Outback Wagon (100K miles) with P0420 Code
I do not know for sure if this is the problem you are having, But, I have noticed that from 95 up to 2002 I have seen a lot of PO420 codes that have been just the sensor. I do not know if it was a weak sensor or just that program telling it that it is below threshold. I did notice that if the vehicles are run for a long trip at highway speed it goes out. I wish you luck and have a great day.
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