richrd0001

What might be the problem after I've replaced O2 sensors but I'm still getting Codes 1 and 2 from ECU and engine is hesitating (bogging?) before CEL light comes on?

I replaced my O2 sensors (both on exhaust manifold under hood as you know, I'm sure) at about 425,000 miles. No problem with them at the time. Changed them because a mechanic friend said it was a good idea. Wrong.

Engine runs smoothly after initial start but gets rougher till it warms up and I've been driving about 5-10 minutes on the highway. When CEL comes on there's an immediate increase in power and engine smooths out.

Initial set of sensors were not Honda, Denso, or NTK. Advised by different mechanic to replace with OEM-quality sensors. Used a pair from a junked '89 Accord on the assumption that an '89 Honda sensor would usable in any '89 model. And $22 seemed a lot better than the $300+ that the dealer wanted.

Advised by same mechanic that sensors are not interchangeable. I ordered correct Denso sensors (234-1006 and 234-1017) this time and installed but I'm still seeing the same problem. If anything, engine is a little less smooth after the CEL comes on now. Problem appeared within 10 minutes of installation.

PS -- The second set of sensors (from the Accord) had a film of carbon that wiped off easily enough after only a few hundred miles of use. That wasn't the case with set # 1. No carbon, I mean. I'm guessing that the carbon is from the fuel mixture being too rich. Got 29.5 mpg with the second set even though wrong ones. Historically I expect about 31 mpg on the highway. I guess the default setting after the CEL light comes on does a good job.

holmesnmanny

If the oxygen sensors are new and you're still getting the code it can only be the wiring or the ecu. Try tracing the wiring to see if it's cut anywhere or maybe rubbing against the exhaust somewhere. That's an issue with some cars. If they look good, it would be best to remove the ecu connectors and test for continuity while wiggling the wires around at the oxygen sensor connectors, down to the oxygen sensor connectors. If that checks out, try another ecu. You could try taking your ecu covers off and looking for burned spots too. I assume the oxygen sensors are in the correct positions.

richrd0001

Thank you very much for your useful response. I appreciate your taking the time to respond very much.

I will try what you suggest. Your last sentence was especially useful because I had indeed put the sensors in the wrong positions. I installed the left sensor on the left side of the engine as I faced the engine instead of installing it on the left side of the car facing forward. Alas, the same problem exists now that I have installed the sensors in the correct position. (I'm puzzled why the sensor that is now on the left side has a much shorter wire lead. I had to slip the plastic connector off of the mount on the block so that the lead from the sensor could be plugged in. It suggests to me that my "mistake" actually had the sensors in the right position. Odd but perhaps the sensors are also used on other engines with different configs. In the wrong position, the different lengths of both the sensor leads matched the different distances to the connectors . . . .)

One thing happened when I cranked the engine after switching the senors which never, ever happens. The engine wouldn't start right away and I had to crank it a lot before it would begin to catch and only after several such attempts did the engine begin to run for longer and longer periods till it finally caught and ran.

Once the engine caught, the CEL light came on immediately and I got ECU error codes 1,2, and 6 ("engine coolant"). I'd not seen 6 before. Once the car warmed up, it ran as before and I had no more trouble starting it and did not see that code 6 again. I jiggled the wiring harness to get more slack to plug in the short-lead sensor. Perhaps that temporarily disturbed the lead from the coolant temp sensor . . . .

However, on the strength of this new code and the trouble starting, I am wondering if a bad coolant temperature sensor might have something to do with my problems. Codes 1 and 2 don't come on immediately when the engine starts, only after I've been driving on the highway for 5-10 minutes. As before, the engine gets rougher as the coolant temperature rises and then suddenly smooths out as soon as the CEL comes on and the codes appear. The engine is smooth as can be when the car has just been started and no codes 1 or 2 are displaying when that is the case. It's only when the engine starts to warm up that it starts to get rough.

To test the theory that the coolant sensor might be bad and the source of the problem, I have ordered a new one to install day after tomorrow. Contrary indication: no code 6 is displaying. We shall see.

I will check the continuity and ECU as you suggest after I try this other idea.

Thank you again.

holmesnmanny

Just a thought. Your engine coolant sensor could be so bad that it's forcing the engine to dump fuel in which is causing your oxygen sensors to read incorrectly. I've actually never heard of this before but it's really the only thing I can think of short of your wiring being bad.

You could also remove the connectors to the oxygen sensors and test the connector pins with a multimeter before even going after the ecu. I'm not sure if you got the 4 wire or the 2 wire. If you got the 2 wires it'll be even easier. If you got the 4 wire one will be 12 volts switched. One will be ground, one will be signal(i think .5 volts) and one will be for the heater element. You can do a search to find out. Just turn the ignition on without starting the car while testing.

richrd0001

Thank you very much, Manny. Forgive my seeming bad manners in not replying sooner.

You were correct about both possible problems. The coolant sensor was ok but I found that one of the wire leads from that sensor had broken. I repaired that break and car went back to its familiar habit of starting instantly.

I also crimped the connector at the end of the lead from the O2 sensors to make it grip the blade of the connector at the end of the wire leading down to the ECU. You'd think I did it enough to solve the problem but I kept getting erratic codes from the ECU. Sometimes indicating one bad O2 sensor, sometimes the other, sometimes both. I finally worked harder at crimping the connector as above and that solved the problem. No more codes and MPG leaped from 28 to 33.7.

Thank you so much for your detailed answers and for your courtesy in responding to my question.

holmesnmanny

no problem at all