Here's one I haven't seen before. Was bypassing a plenum-mounted det sensor as a possible culprit. I suspect it's been tripping the ECU into safety boost judging by partially fouled spark plugs. The det sensor itself looks as-new and the wiring was intact (continuity tested). Here's the rub: No error codes. Bypass det sensor with 1Meg Ohm (checked) resistor. Verified a good fit with a continuity test on both sides of the wiring plug. Run diagnostic again ... 34 . so recheck the resistor installation, still 34. Try another known good resistor ... still a persistant 34 (pops up straight after clearing it) like an open-circuit condition. Suspected a broken wire in det sensor circuit from disturbing the wire so reconnected the det sensor wiring loom to the det sensor ... = code 55 WFT. I retry another resistor ... 34, replace the wiring loom to the det sensor ... 55. The resistors Ohm out at 1Meg Ohm and are part of the same batch I bought a few weeks back and I used one on another car with success. I continuity recheck the loom with the resistor in place. All checks out 100%. I try wiggling the loom to reveal a loose wire ... no difference. So the ECU is happy with the det sensor resistance, but not with a 1 Meg Ohm resistor (which is the default replacement resistor to bypass code 34 and fool ECU into thinking a working det sensor is in place.) I sent the owner off to buy some different resistors but I suspect he will have the same problem. I plan to run a wire direct from ECU pin 25 to a chassis earth with a 1Meg Ohm resistor in-circuit if required. There must be some gremlins in the engine bay wiring loom perhaps, but you'd think this would trip a 34 regardless of whether the det sensor or resistor is connected. The det sensor was already installed so no way to know if it is the correct one (but if it wasn't, the ECU would throw a 34.). The ECU is stock, unmodified. Ideas? EDIT: Just occured to me that the coaxial earth wire may be touching another wire somewhere. When connected to the det sensor (earthed onto the plenum, it completes the circuit), but with resistor, the earth connection is removed, hence the code 34 ...
I think you may be right.... Put the multimeter over the coaxial and see if you have a short somewhere. Are you laying the harness in a different position when fitting the resistor ?
What the ohm reading of the sensor ? . get a reading then if it's not 1 meg ohm put a resistor that matches the reading you get just for the fun of it to see what happens. Cheers Dave
Do the bypass at the Ecu, about 8" from the ecu connector, the 2 det sensor wires join into the coax, fit the resistor there. I had one doing the same last week and this fixed it. I have found alot of wiring harnesses becoming very degraded where the sharp bend is near the IACV, most of them showing weird intermittent problems.
I'm no expert, but my understanding is that the normal return path for the det densor is through the sensor body to the block. With a bypass resistor the return is via the cable screen, which is separately earthed. If the cable screen isn't earthed properly, a working sensor will give a code 55, but the bypass resistor will give a code 34. Just a theory.
