P0420 does not necessarily mean oxygen sensor. While it is true that O2 sensor will indirectly trigger the code, there are other parts that directly cause it. Here is a snapshot of P0420.
P0420 OBD-II Trouble Code Technical Description
Catalyst System Efficiency Below Threshold (Bank 1)
What does that mean?
The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
Symptoms
You will likely not notice any drivability problems, although there may be symptoms.
Causes
A code P0420 may mean that one or more of the following has happened:
* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible Solutions
Some suggested steps for troubleshooting a P0420 code include:
* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* Use fuel with manufacturer recommended octane rating.
* Inspect coolant temperature sensor.
* Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
* Inspect the downstream heated oxygen sensor (HO2), replace if necessary.
* Replace the catalytic converter.
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.
** This is what I would do. Reset the PCM. If the light comes back, then you know something is faulty, and needs repair. **
Dec 15, 2008 at 5:14 PM
