What causes a code 1 to be set for something other than a faulty O2 sensor ? Two new sensors later, I am still intermittently setting code 1 after the warmup period. I have traced out and tested the sensor wiring up to and including the ECU and have swapped ECU's - same problem. Checked engine ground connections, no problems found. 1991 4WD wagon with D16A6 engine.

 

Could be the O2 sensor wires (Ohm test) or the ECU.

 

O2 sensor wires have been tested with an ohmmeter from the actual sensor wire all the way through the wiring harness and all connectors to the connector on the ECU. I have connected a digital voltmeter to the O2 line internal to the ECU and drive tested while watching the O2 output voltage - it swings from zero to a maximum of about 0.9 volts during drive testing depending on speed and throttle position. In other words it seems to be working as it should.

 

Did you both continuity and short test the O2 sensor wires?

Your "ECU test" does not rule out a bad ECU.

 

Yes, the O2 sensor wire has been rung out and tested for open and/or ground /shorts with no unusual problem detected. I have checked all of the vehicle ground connections as well as the ECU ground. The connector pins in the harness have been checked for intermittent problems by wiggling them while watching the ohmmeter end-to-end. The contact pins have been cleaned of debris and corrosion. The ECU has been swapped with another working ECU - I have total of 5 ECU's and 2 running cars to swap parts between. I can swap ECU's again but I just want to be sure that there is nothing else that can set a code 1.

 

The service manual states that a faulty fuel pressure regulator can also throw code 1.

 

Check the second trouble section for code 1
Diagnostic Code Charts - TheZCR.com Forums

 

Good tip on the fuel pressure regulator -Thanks - I will perform those tests before resuming the hair-pulling.

 

Assuming the ECU has been reset after installing the new sensors?

 

The check engine light resets every Key Off-Key On cycle, but retains the code. I generally disconnect power from the ECU after each test which clears the code also.

 

Having the same problem with my 89 Si. Have reset the ecu, checked the fpr, wiring, sensor, thermostat housing ground, every other ground as well. The only thing I haven't done is swapping the ecu, I hate to buy another one just to hope that's it. Having some driveability issues also. Any of those on your end? Lack of power/stumbling when driving, usually partial throttle. If I turn the key off and back on while driving the problem goes away for a length of time, dependent on driving conditions. Subscribing. If I come up with anything I will let you know also. Are you using Denso sensors?

 

I'm using Bosch universal replacement sensors. I tested the last one I removed in the shop by hooking Digital voltmeter directly to sensor and using propane torch to heat, then measuring voltage swing as I passed torch over the sensor. FWIW I have 5 ECU's to swap around , but the 2 I've used so far have identical symptoms so I really doubt if that is the problem. I am waiting on a part to connect a fuel pressure gauge directly to the fuel rail by replacing the cap on the end.

 

If you don't mind let me know what you come up with in regards to connecting there. I connected at the fuel filter test port when I checked my fuel pressure, and the fpr seemed to work properly, testing it with vacuum unhooked as well as doing the pinch test on the return line. My next step is to bring my injectors to a local place that does ultrasonic injector cleaning and bench testing, I figure that probably isn't the issue but it couldn't hurt on a higher mileage motor. Hoping that will smooth it out a little bit at the very least.

 

Once I have completed the fuel pressure test I am going to install a 3rd ECU. My injectors have been recently removed and cleaned but I have a spare set if needed, though I doubt that's a problem for me as my plugs all show normal combustion and I would think a stuck or dirty injector would show up on the plugs as super lean or carbon fouled.

 

Update - I have tested the fuel pressure regulator and it appears to be operating OK (See Pictures). I have replaced the ECU (3rd one so far) to see if there is any change.

https://dl.dropboxusercontent.com/u/7848444/1.jpg

https://dl.dropboxusercontent.com/u/7848444/2.jpg

If I still get a code 1 then I'm fresh out of ideas.....

 

The fuel pressure is below spec both with and without the regulator vacuum hose disconnected.

 

Thanks, I stand corrected. More troubleshooting required.

If all else fails, Read the Manual

 

My fsm specifies 35-41 with vacuum disconnected from regulator

 

 

I replaced the pressure regulator with absolutely no change. Fuel filter is new.

 

Did the fuel pressure readings change? The fuel pressure regulator is only one possible cause for the pressure readings being out of spec. See post #16.

Directly from the 1991 CRX (ED9) service manual:

 

This^ is true for the 1988 CRX (ED9) service manual.

 

Mine is an 88 Civic Honda FSM. In going to go back and double check my fuel pressure with a different guage

 

With everything hooked up properly, does the fuel pressure drop off when you turn the car off and let it sit?
Or does it hold pressure for a very long time?

 

Had the same issue awhile back. The cause of it was the way someone repaired a broken engine harness plug. They spliced end of plug back together with a male and female spade connectors. Which was causing too much resistance. De-spliced them and twisted them together. Insulated them real well. Problem solved. Just sharing my experience. Not sure if this helps or sparks new theory.