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96 civic auto, 130k, good mechanical condition. No issues. Replaced spark plugs and wires and pcv valve, and cleaned entire intake tract and idle aire control valve 4k ago.

This started last week. CEL stays solid only when engine runs. Turn it off and it goes away. Doesn't happen all the time.

I shorted the blue SCS connector and turned ignition key to run position (with engine off) but get no CEL flashes. So whatever the cause of the CEL it does not get stored.

Safe to assume that misfires cause only a flashing CEL?

What could be causing a solid CEL that does not get stored?

Can anyone recommend a cheap scan cable that will help me view the code posted with the engine running? The cheapest obd2 reader I can find around here is $55 and over. Anyone in the SF bay area know of any deals?

Thanks for any help.

 
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Can I short the SCS connector to read the codes while the engine is still running?

I read that shorting the SCS with the engine running is prescribed when setting the timing, but I'd like to make sure it won't reset or disrupt anything else with the ECU.

I'll restart the engine without the shorting clip when I am done.


for reference:
http://honda-tech.com/honda-civic-de...lists-1901557/

 
Rep Power: 519

Google shopping?

 
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I was able to get the code with the engine running, by shorting the SCS connector. I got code 41, pri. oxygen heater.

It sort of makes sense with the behavior I am seeing. The CEL comes on after a cold start (high idle) and stays on - heater tries to get the sensor warm enough to go into closed loop faster. If it is restarted when warm (coolant temp sensor input to ecu?) presumably the O2 heater is not needed so the CEL doesnt come back on.

I'll measure resistance at the heater, check wiring, fuse, relay and see where the fault is.

I will have to pass California smog. I usually bring it in hot before a test but I wonder if they do anything to force a heater test. Does this affect readiness monitors? I'll need a real scanner for that.

 
Rep Power: 519

Time to set P0135 code is 5 seconds in fault condition

Sensor could be affected by temperature, heater coil inside probably makes connection when hot

Monitors all reset when codes are cleared or battery disconnected.

CEL can go out after 3 good self tests but code and freeze data will be stored in memory for longer than that, so you probably will want to manually erase the codes and drive it until the monitors all pass..

 
Rep Power: 211

Scanazon:

 
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Update time.

for the O2 sensor, unplugged of course:
- pins 3 and 4 (black wires) on primary sensor show 15-18 Ohms, i.e., within specs (which are 10-40 ohm, per manual).
- no continuity between 3 and ground, or 4 and ground

So, according to the Factory Service Manual troubleshooting chart, the sensor is fine. (I did take a pic of some numbers on the pri O2 sensor -- 288, even though I couldn't finagle the heat shield out of the way).

Some initial testing on the female connector side showed the possibility of a short or drain. I was getting about .10 volts lower on the battery with the key on v. off, and another .10 volts lower when I tested voltage on pin 4(*) and ground. I didn't measure current draw between key off and key on.

I reset the PCM and the CEL went out but aftet a few drive cycles it is back. I'll test the sensor again and try to track down electrical faults.

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(*) for anyone else investigating similar issues, pin 4 on the Pri O2 Sensor connector is where you test for battery voltage on the D16Y8 engine. It's pin 3 normally. The troubleshooting diagram in the manual has lots of little asterisks where exclusions and exceptions are stated.

 
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I am stumped.

I think I will need to keep resetting it and keep a multimeter in the car at all times so I can measure the resistance as soon as it fails. That is assuming the internal heater is intermittently faulty.

 
Rep Power: 519

If CEL comes on right after a cold startup, then measure sensor resistance before you start the engine. Sensor stone cold.

 
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The measurements I took were with the sensor cold (car sitting in garage for a day). It was about 18 Ohms. I measured it again after a 7 min drive, and it went to 27ohm. It sat for about 15 min, and was driven again for another 20 min, and then the resistance was at 55-60 Ohms. I checked for shorts between pins 3/4 and ground, and found none.

Is the internal heater resistance supposed to change that much with temp? In my case, we are looking at a range of 17-60 ohms, the manual says 10-40 ohms, which I assume is across all temps.

I don't normally throw parts into my repairs, I'd like to reproduce a fault reliably well before replacing a part. Should I remove the O2 sensor to read the numbers off it to get a replacement?

Also, after the longer drive when I was measuring resistance, I turned the ignition on, and there was no voltage on pins 3-to-ground, or 4-to-ground on the harness side of the connector. Can I assume the the ECU does not turn on the O2 heater when the engine temp is above some threshold?

Somewhat unrelated, with the key in the on position (engine off) I noticed a faint buzzing sound coming from below the back of the radiator fan motor housing. I stuck a tube and tried to isolate the source further, with no luck. I didn't notice the sound when I worked on the car before (when the engine was cold). I'll try with a metal rod next.

Thanks for any help on this. I am becoming familiar with hondas via this car. I have done way more with other imports of similar vintage.

 
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One more thing. Given the climate here, temps don't normally get below 45 F in the winter, so the only way to get the O2 sensor colder would be an ice pack on it or something like that.

Cars here usually come up to temp within a few min of driving, even on a cold morning.

 
Rep Power: 519

Sensor is junk. Replace and see how it drives afterward.
I'd at least check and research the original brand, if it's still original. Denso or NTK.

ECM controls the ground side of the heater circuit, and I can't tell you if it's operated based on temperature but that would make sense.