Got this 1999 Honda P0420 code on customer’s Odyssey and it is by itself. Meaning there are no companion code for the oxygen sensor. According to customer, the vehicle ran fine since the last 2 months he bought it but the dash check engine light started coming on. He is not using any fuel but just concerned on the long term effect of the code.
Diagnostics on 1999 Honda P0420 code:
When I saw the vehicle, I started the engine from cold and the check engine light was on all the time even when the engine has warmed up. Scanning for codes, I got P0420 which means catalytic converter has low efficiency. The vehicle has about 135k kilometers and other than this code, the engine was running fine. I scrolled on the live data from the scanner and here are the voltage readings I got from the oxygen sensors:
Upstream oxygen sensor (located in exhaust manifold)
0.2v to 0.8v which shows normal
Downstream oxygen sensor (located in the catalytic converter itself)
0.075v to 0.085v which shows below normal and almost no fluctuation
Note on 1999 Honda P0420 code:
The engine has 2 kinds of oxygen sensors: one for the engine management called upstream and one to monitor the catalytic converter performance called downstream.
Both of these sensors have a pre-determined voltage readings and if the readings are not within range, the computer will flag a light in the dash panel. In most other cars I have worked on, the downstream oxygen voltage fluctuation range from around 0.4v to o.7v and the reading I got was pretty low.
Solution for 1999 Honda P0420 code:
I would replace the downstream oxygen sensor first and try it for a few days until the code returns. If the code does not return, I would say the problem is definitely the oxygen sensor. The catalytic converter on this is almost about $1000 from the dealer and I hate to recommend it and later found not to be the culprit.
Caution: This 1999 Honda P0420 code is just a suggestion, make sure to check with your local licensed Honda mechanic to verify the solution or leave your comments below. However, for maintenance purposes, the oxygen sensor must be replaced at 100k kilometers.