After months of normal driving with no problems, my 2003 Honda Civic EX check engine light came on during a long trip. Got it scanned and it's P0420: Catalyst efficiency below threshold bank 1. They said it could be a bad catalytic converter or oxygen sensor.
I borrowed a scanner and tried to measure the oxygen sensor readings over time. Doesn't seem to have good time resolution:
This is voltage vs "time", though I don't know what units "time" is in, or if this is useful in any way. The code did not occur during this graph, anyway. Apparently the sensors are supposed to switch on and off like this?
The lambda sensor's output voltage should not remain constant, however. It should flip-flop back and forth from rich to lean as the PCM increases and then decreases the amount of petrol injected into the intake manifold. Every time the voltage reverses itself and goes from high to low or vice versa, it's called a "cross count." A good O2 sensor on an injection system should fluctuate from rich to lean about 1 per second. If the number of cross counts is lower than this, it tells you the O2 sensor is getting sluggish and needs to be replaced. http://www.fordscorpio.co.uk/obd2scan2.htm
Though another site says:
Once a fully functional catalyst is up to working temperature and is “lit”, we should see a nice “flat line” signal from the downstream sensor, which shows that the exhaust gases have been successfully converted. http://www.picoauto.com/tutorials/cat-efficiency.html
An example of a single frame:
MIL STATUS OFF ABSLT TPS % 25.4 ENG SPEED RPM 3018 CALC LOAD % 74.9 MAP KPA 75 COOLANT °F 194 IAT °F 109 IGN ADVANCE DE 21.0 ST FTRM1 % 0.8 LT FTRM1 % 3.9 VEH SPEED MPH 20 FUEL SYS 1 CLSD FUEL SYS 2 N/A O2S11 V 0.875 ST FTRM11 % 3.1 O2S12 V 0.780 ST FTRM12 % -1.0 OBD2 STAT CA Frame 21 Time 74.3
I cleared the code and it didn't come back during normal driving, but then during the long trip in the opposite direction, the engine light came on again. Both times the light came on were shortly after filling the tank and getting back on the highway. (I measured 38 mpg during this trip, which doesn't sound like anything is seriously wrong with efficiency.) I assume it's the same code this time but haven't read it yet.
It passed emissions inspection 02/19/2011.
I have oscilloscopes and volt meters if that would help figure out the cause.
Update: I replaced the upstream oxygen sensor and the light still comes on. It comes on after filling the tank with gas, and goes off after filling the tank with gas. So something about the gas being slightly different from one station to the next?