I have a Honda Prelude 1994 with a Honda P14 ECU and I’m going crazy trying to solve a persistent Code 41 (O2 Heater) issue.The car actually runs perfectly fine: smooth idle, drives normally, no noticeable misfires, fuel consumption seems normalBUT:the check engine light stays on constantly only Code 41 is storedWhat I’ve already done: swapped the ECU with another P14 replaced the O2 sensor tested the O2 heater resistance → about 16 ohms. followed the entire Chilton troubleshooting procedure still inconclusive
Now comes the weird part:If I COMPLETELY unplug the O2 sensor connector:the error remains EXACTLY the sameonly Code 41NO Code 1From what I know about Honda OBD1 systems, I would expect:Code 1 for missing O2 signalor Code 1 + 41 togetherBut it keeps showing ONLY Code 41.Another very strange thing: Using hondash the O2 reading stays fixed around ~3.8V, even though a narrowband sensor should fluctuate roughly between 0V and 1V.NOW HERE IS THE REALLY CRAZY PART: If I place a multimeter probe directly across the O2 sensor A+B pins:I get around ~0.43V to 0.45Vand AT THE SAME TIME HondaDash suddenly “corrects itself” and starts showing a realistic voltage tooSo somehow: the multimeter itself seems to change the circuit behavior.That makes absolutely no sense to me.At this point I’m starting to suspect:wiring harness leakagebad groundingweird ECU pull-up behavioror an issue in the O2 signal line itselfQuestions:Has anyone ever seen a Honda OBD1 system keep ONLY Code 41 even with the O2 sensor completely unplugged?Does the constant ~3.8V point more toward a short/leak in the harness?Could the multimeter “fixing” the reading indicate a grounding or impedance problem?Is it worth rewiring the ENTIRE ECU → O2 sensor harness from scratch?Pinout:A6 = heater ground control (Orange/White)Green/WhiteWhiteBlack/YellowCan the Black/Yellow wire safely be temporarily connected to a reliable switched 12V source for testing?Any deeper technical insight would be greatly appreciated because this failure behaves completely outside the normal Honda OBD1 patterns I’m familiar with.