My daughter has a 2000 Accord 4 cylinder vtec. The MIL was lit, so I hooked up the scanner. I now know that I have a P0135 code which says "Front HO2S Heater Circuit Fault (Sensor 1)". I tried search to no avail. My question is, now what do I physically change out or check to fix this problem? Any help would be greatly appreciated.

 

You should first check that there is voltage to the heater circuit of the sensor with the key on. 2 wires the same color [usually black] are the heater circuit.

The heater circuit of the sensor itself should have around 20-30 ohms, if my old memory is working today. LOL

 

Thanks hondadude! The message associated with the P0135 code says "bank 1 sensor 1". Is there more than 1 O2 sensor? If so, which sensor do I check based on this information? Where is it located? Thanks.

 

It's in the exhaust manifold. Between the radiator fans and engine block. Look down and it's on the left.

There are 2 O2 sensors, one where I said and one after the catalytic converter.

 

Thank you!

 

Well, I checked for 12 volts at the connector & had 12 volts at one of the pins with the key turned on. I changed out the sensor & reset the light with my scanner. The light comes right back on. Same P0135 code, O2 sensor heater circuit, bank 1 sensor 1. I'm kind of at a loss now.

 

If you're absolutely sure you had 12V on one of the O2 heater leads, the O2 sensor is likely befaulty. If you check resistance it should show the specified resistance. If open or short, the O2 sensor is defective and should be returned.

I resistance checks good, another possibilty is a faulty O2 sensor electrical connector(harnessside)preventing the circuit from completing.

good luck

 

I'm not absolutely sure. All I know is that I have 12 volts at 1 of the 4 pins only. This is with the key to "on" (engine not running). I don't have a diagram to tell me which pin is for what. Thanks.

 

If you disconnect the O2 sensor there are 4 terminals, two side/side nearest the latch side and two side/side below. The heater circuit is the two side/side lower terminals. They should have black wires. Test O2 sensor side of connection for resistance between two lower pins(10-40 ohms from 94 Accord spec, yours could be different). If ohms are out of spec, the O2 sensor is faulty.

Test harness side of connection. You should show 12V between the two lower side/side pins. If no voltage, then harness or ECU problem. If you have voltage, then electrical connector is somehow not mating to O2 sensor connector, if O2 sensor is OK.

Trying todo this w/o a manual is like flying blind w/o instruments, you don't know where you're going.

good luck

 

A little more info: I found that the first O2 sensor I changed out was for a F23A4 engine (California emissions, I think). Mine's a F23A1. I don't know if this would matter, but it did not fix the problem. I've got a wiring diagram and have confirmed power to the heater circuit of the sensor. I get power with engine running when I go from the power pin on the harness to chassis ground. However, when I jump the two pins at the connector, I get no voltage. The diagram shows that wire running back to the PCM, so I don't know if it's SUPPOSED to complete the circuit or if there's circuitry inside the PCM just waiting for a signal.

FWIW, I reinstalled the original sensor and checked the heater circuit. With my DVOM set to 200 ohms, I get a momentary(<1 second) indication of about 1150 ohms, then open circuit. I've been told I should get about 30 or 40 ohms steady on the heater circuit.