I’m fairly handy on a car, but electrical work is well beyond my qualifications. Tried to start my '01 Nissan Maxima, and it wouldn’t turn over despite everything sounding good and seemingly working. Took it to a shop I’ve worked with plenty over the years (and used to trust a lot), and this is the story they are painting:
The rear engine mount is busted (which after looking at it I buy) caused the sensor to fire, which some how cause the ECU to get fried. I want to note they took a full 3 days to come to this diagnosis. I’m buying the rear engine mount problem, but my research into the sensor indicates that the blown sensor shouldn’t prevent it from turning over. I’m concerned that some how the ECU was damaged in another way (not necessarily by the shop or maliciously).
It’s got 144k mi on it, and the only symptom of electrical problems I can think of is that we have a key that only starts the car 80% of the time (and 1 key which previously always starts it). I understand that the key is supposed to send a coded signal based on the VIN to the ECU to get the car to crank, but I’m not sure where along that line it would cause ECU failure.
Seems to me the simplest (and perhaps cheaper) possibility is replacing the ignition switch. You say yourself it has a “mixed” history of actually starting the car, and on a 2001 model I wouldn’t be surprised to see a “tired” ignition switch.
What do you mean “turn over”? Does the starter energize and tun the engine, but the engine doesn’t start? This is usually termed a “crank, no-start” condition.
What sensor are we talking about? Did the broken engine mount cause some wiring to chafe and short? It is possible that this could cause an ECU failure, but what you need to do before replacing the ECU is to get a wiring diagram for your car and go through all of the related sensor wiring and make sure you don’t have any shorts. There are sensors that need a reference voltage to work, and sensors that have a fairly high current applied to them, like for the heating elements in heated oxygen sensors. Note that the sensors themselves can have internal shorts as well. If you have a short in either of these, it will cause the fuse to blow, hopefully before damaging the ECU. You might be better off turning this one over to a shop that is known for electrical diagnostics and/or familiar with your model of vehicle, if you haven’t already.
I don’t see how the engine mount sensor could fry the ECM. A simple test is to unplug the mount sensor, then replace the fuse and see if it blows upon turning the key on. If it still does your problem lies elsewhere.
@oblivion I’ve already taken it to a shop, as I mentioned, it was there opinion that the rear “crush sensor” on the rear engine mount had fired due to the rear engine mount failing.
@Cavell I did now. My understanding is the original was OEM.
@asemaster Looking to do that after we get a new ECM. The ECM is toast.
I agree with @oblivion. The real problem may be within the wiring after the ECU. One simple thing could be checked. Check the continuity to ground from the fuse position that is blowing out while the ECU is installed. Since the fuse blows out there should be a very low resistance on that power supply wire going to the ECU. Then disconnect the ECU and measure the resistance again on the pin of the ECU the power wire goes and to a ground pin. If the resistance is still low then there is an internal problem with the ECU. If the resistance is now high this would seem to indicate that the real problem is after the ECU, possibly a sensor power wire as was previously mentioned.