The malfunction indicator lamp has been on in my '03 Honda Civic LX for some time now. The car shakes and has trouble accelerating, but not all the time. This usually occurs when the car is stopped, perhaps at a stoplight, in drive. The malfunction indicator lamp blinks when this happens. I brought it to have the codes read, and these came up; P0303 (this happened twice in one reading), P0420, and P0135. Has anyone got a clue?
Code P0135 = O2 Sensor Heater Circuit Malfunction( Bank 1 Sensor 1).
Code P0303 = Cylinder 3 Misfire.
Code P0420 = Catalyst System Effeciency Below Threshold (Bank 1).
The first code indicates that there’s probably a problem with the O2 sensor.
The second code indicates that cylinder 3 has a misfire. This could be caused from a bad spark plug, a bad spark plug wire, and so on.
The third code is more expensive. This code indicates that the catalytic converter is no longer capable of preventing pollutants from entering the atmophere.
When the Check Engine light started flashing, and you kept driving the vehicle, it meant that the misfire in cylinder 3 got to the point where damage to the O2 sensor and the catalytic converter were going to occur.
It looks like the thing to do is figure out what is causing the P0135 code. Once that is fixed, the other ones might go away or at least – in the case of the cat code – the engine might return to running correctly.
So what is causing the O2 sensor heater circuit problem? Could be the O2 sensor is bad, but it might be the wiring to the circuit is faulty, or the ECM circuitry which controls the the O2 sensor is faulty. There are tests a mechanic can do to determine which of these i sthe problem. The first thing they’d probably do is measure the resistance of the sensor’s heater element.
There is a possibility that the cat is plugged and that is what is causing this problem. A mechanic could prove or disprove this by testing the back-pressure, or just temporarily disconnecting the cat and see if the performance problem went away.
Fix misfire first
Then measure backpressure
If it fails that test, the cat is plugged because you ignored the misfire too long
P0135 is almost a no brainer. Unless there’s damaged wiring, the sensor is probably faulty.
This would be my order of priority