If That Fuse (#15) Is Blown Then It Would Behoove You To Search For And Obtain A Copy Of A 2-Page Honda Technical Service Bulletin #99-029, Written To Help Honda Technicians With Some 96-99 Civics With Problems Involving PO135 (Or A Couple Other Codes) And A Blown #15 Fuse.
The fix is fairly simple, locating a damaged wire and insulating it. It should be fairly inexpensive to have done. I would think anybody who works on Hondas on a regular basis would know what this is about.
CSA
CSA- is that what the P0135 code is indicating? Thanks again!!!
Also CSA- just wondering, if my brake fluid is low, is that something I could do myself? Or do you still have to bleed the brakes? I am not looking to change out the fluid (unless I should) but add to it. Thanks so much!!! You’ve been an amazing help. I was up all night crying over everything going wrong with my car. It is so overwhelming. I am trying to soak up as much information as I can. Thanks again!!!
P.S. Do you live anywhere in the midwest? If so, I’d gladly bring my car to you 
The Code Is Indicating A Fault In The O2 Sensor Itself OR A Problem With The Circuit That Wires The Sensor To The Car’s Engine Computer.
You could blindly replace the sensor and possibly not resolve the problem. It seems that there have been a number of these Civics (every kind of car has idiosyncrasies) that through rubbing of wires against a bracket, develop a bad spot in the wire (circuit) for the O2 sensor. It could be that the sensor is bad or it could just be the wiring.
Checking fuse #15 and finding it blown, along with the PO135, helps lead one to believe that a wiring problem possibly exists. Replacing the fuse without looking at and repairing wires would probably just result in another blown fuse. Fuses blows to protect items on a bad circuit and helps prevent fires, etcetera.
Again, it could be a bad sensor or it could be the circuit for the sensor. Trouble codes can’t tell you which it is. Many people start out with the expensive guess and replace parts, first. Why not have a look at that fuse? Takes 2 minutes. Then download a copy of the bulletin and have somebody check the wires before replacing a sensor.
I had an 02 code on one of our cars (not a Honda) , once. When I looked to see where the sensor was, I found the problem. The wire leading from the sensor was literally broken off.
CSA
Oh, wow, ok thanks!!!
Here’s what I would do concerning P0135 . . .
Turn on the ignition
Measure available power at the heater circuit for that oxygen sensor. If it’s good, the heater circuit is almost certainly bad.
Aka . . . you need an oxygen sensor
If there’s power, then you can forget all about the blown fuse and that bulletin
concerning your low brake fluid level . . .
put your car on jackstands
Look for brake fluid leaks
If none are found, look at your pads, shoes, rotors and drums. In all likelihood, it’s time for a brake job
When your brake pads and shoes are worn out, the brake pistons move further out, displacing ever more brake fluid. That means there’s less fluid in the reservoir
Generally speaking, when a brake fluid reservoir is low, there’s a good chance you need a brake job
fyi, many O2 sensors actually have two circuits inside, the actual O2 sensing circuit, and a separate heater circuit. The heater circuit is just an electrical resistance heater, the same idea as you’d find on a heating element for an electric stove. The purpose it to quickly heat up the O2 sensor after the engine is started. O2 sensors aren’t accurate until they get hot enough, and the faster they heat up, the sooner they can be used to reduce the emissions. My 20+ year old Corolla for example uses an unheated O2 sensor. But the emissions rules folks and the manufacturers discovered they could reduce auto emissions further by using electrically heated O2 sensors.
A heater circuit has to have very low resistance in order to be a good heater. The computer is able to measure the resistance, and apparently yours is measuring too high. Or it is measuring as a direct short to ground. Either way, something’s amiss. As mentioned above, this could be caused by a faulty O2 sensor, faulty wiring, faulty connections, or a fault on the circuit board that controls the heater circuit. If you have the pin-out of the sensor, you should be able to measure the resistance of the sensor’s heater with a DVM. What does it measure? Do you notice any signs of charring on the terminals inside the connector?
“Also CSA- just wondering, if my brake fluid is low, is that something I could do myself? Or do you still have to bleed the brakes?”
db4690 has given you some information concerning the brakes, although I don’t think you’ll be putting your car up on jack stands and I don’t recommend you try that by yourself.
But, as was pointed out, the fluid can go down a bit in the reservoir through normal brake wear. An empty or nearly empty reservoir and/or braking problems could indicate a fluid leak that needs to be immediately addressed. It could be DANGEROUS.
Befriend somebody who could show you how to check the brake fluid reservoir or ask the parts guys/gals. It is very easy if you know how, takes seconds. It is also easy to add a little fluid if you know how. Use only the proper brake fluid, probably DOT3. Brake fluid will remove paint if spilled on the car’s finish, so be careful.
Please explain why you are "just wondering, if my brake fluid is low…"
Are you having braking issues (stopping distance too long, brake pedal going down too far, car pulling to side when braking, etcetera)? Please explain.
CSA
Speaking of OEM parts, I once replaced a bad Denso oxygen sensor with a Bosch one. I figured Bosch makes a good product so why not go that route? Anyway, I had recently replaced the engine. The old one was a real oil burner. I mean a REAL oil burner. The clouds of smoke were actually stopping traffic and I pretty much had to add a quart of oil each time I drove it. That engine was toast.
As you can imagine, the O2 sensor was shot after all this. I replaced it with a Bosch and the codes for a bad O2 sensor returned. I read on the forums for the car in question that you ONLY want to use a Denso sensor as using anything else can cause issues. I swapped it out with the Denso and my problems went away. Driveability and fuel economy improved. For certain things, using the OEM part seems to be VERY important. I can’t see a gas cap being that big of a deal as that is a low tech part but maybe some of the aftermarket parts are just cheap. I had to replace some vacuum elbows on my Ford truck that the mice ate. The new ones dry rotted in less than a year so not all aftermarket parts are quality. I think you should have the codes cleared and see what happens with the O2 sensor code. If you do discover it needs to be replaced, I suggest the OEM variety.
Another thing mechanics have ALWAYS told me to buy OEM is the fuel pump. One told me he would refuse to install the “parts store” brand of any fuel pump and would insist on the OEM version. The OEM ones often cost quite a bit more but I guess you get what you pay for.
Also, There Is No DTCode For “Gas Cap”. Other Faults In The EVAP System Can Be The Culprit(s).
CSA
True! The gas cap is cheap and easy so I always try it for that code. So far, it has always worked for me.
Last year, an ex-con murderer here in Mexico wrecked several cars when they stopped and he did not. The rear hatch on my Sienna and the rear bumper were pushed in There is a kid here who worked many years on body repair in Phoenix. Cost was around $200 total.
He took the door over the gas cap off, took it to a town an hour or so away, and they matched that paint perfectly.
My wife was furious. When the wreck happened, the guy who didn’t stop, ran up and blamed me for stopping in front of him. Hee, hee. The reason I stopped was a transport van in front had a man fall out into the street in front of us. Mexico has the same rules on that as in the US. You are responsible for the vehicles in front of you, not the vehicles behind you.
The good news was the wreck happened in front of a state office, and a man in side there saw it. So, he talked to the transit investigator.
The ex-con’s mom was with him, and got hurt and had to go to the government hospital. Personal injury wrecks in Mexico are serious things, so we couldn’t move the cars until the state investigator came out from nearly an hour away.