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My Chevrolet Aveo 2007 had P0340 code and crank, but no-start condition. I bought a replacement camshaft position sensor. The P0340 code is not there anymore, but the car still does not start.

First of all, the new sensor that I just bought, presumably has misaligned screw holes, because it kinda leans to the right side more than the old sensor (see red arrow):

leaning to right

I have attached with my oscilloscope to orange and black wire:

Oscilloscope wiring

The output on the new sensor to me looks even worse than from the old sensor, because now instead of regular even pulses:

The old sensor

I am now seeing one long pulse and one short pulse that repeats:

The new sensor

The questions I have:

  1. Am I expected to see nice square waves in osciloscope? Unfortunately, with neither of sensors the beginning of pulse was steep.
  2. Why with the new sensor I am seeing one long pulse? Is it because of sensor leaning to the right and that it does not take into account left gear hence "pulses are merged"?
  3. Why the code P0340 disappeared with the new sensor and does not return, but the car still does not start?
  4. What voltage should I see between black and orange wire? After turning key to ON but before cranking I see how voltage grows slowly over 10 seconds from 0V to 12V. And then when I crank I see waves of 2V amplitude as in oscilloscope pictures I have attached here. This has made me curious that perhaps PCM INPUT pin is leaking electricity?
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  • I cannot tell you exactly how it's supposed to look, but there should be some difference somewhere in the waveform. This is how the computer can tell exactly where the cam is at. If it was all the same, it couldn't tell exactly where in the cycle the cam is located.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Oct 7, 2017 at 22:03
  • @Pᴀᴜʟsᴛᴇʀ2 Found this article where camshaft signal examples are always nice square waves: easyautodiagnostics.com/misc-index/ckp-cmp-sensor-basics-3 I think that new sensor is simply busted because it is leaning to the right side. Whereas in my case both sensors were also lacking steep starting edge.
    – user2138912
    Oct 7, 2017 at 22:15
  • Okay, I think I misspoke. I can see how the waveform should be the same. I think what I was getting at is the frequency and width of the modulation will be different. At this point, I'll bow out of this because I'm well beyond my knowledge level.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Oct 7, 2017 at 22:42
  • I'm not familiar with this specific unit, but in general, if the new part is positioned differently than the old one, I'd check to be sure it's not installed backwards, and that it's the correct part for your car. It would be very unlikely for a mass-produced part to simply have the holes in the wrong place. Can you compare the old and new parts?
    – barbecue
    Oct 8, 2017 at 3:58
  • Also, the square wave output shown on the page you linked is specifically for a hall-effect sensor. It's possible your sensor is magnetic, or is designed differently. You'd need to compare the waveform to one from a known good sensor of the same type on the same model car.
    – barbecue
    Oct 8, 2017 at 4:04

3 Answers 3

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I'm gonna take a stab at answering some of your questions, even though I'm not familiar with your vehicle.

  1. A perfect square wave may or may not be normal for your car, you'd need to know what type of sensor it is, and how it's designed to work. I couldn't find anything specific about this model.

EDIT: Based on your latest comment, I would be strongly inclined to believe you were given the wrong sensor. Amazon is notorious for shipping incorrect parts and counterfeit parts, due to the huge number of providers they work with, and the third-party suppliers who use Amazon as their sales venue.

  1. Seems reasonable.

  2. P0340 is a generic error indicating a problem with the cam sensor circuit of cars in general, it's not specific to your model. A circuit error usually means a short or incorrect voltage is detected. The code was triggered by that defect. If the replacement sensor does not have the same defect, it won't trigger the same error code. Basically, the voltage could be correct even if the sensor not the right one or is in the wrong place.

  3. 2 volts doesn't seem unreasonable to me, I think the circuit's only 5 volts total, but again, I dunno the specifics.

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  • thanks. I updated my question #4. Basically when I turn key to "ON" I see in voltmeter how voltage between Ground and sensor's orange wire grows from 0V to 12V. Then, when I crank, this voltage goes back to 0V and up to 2V only on peak sensor reading. Is it likely that ECM is actually bad and the INPUT pin is leaking current, hence it slowly grows voltage to 12V when not cranking and when cranking it prevents steep rising edges? Since EE is not my background, then I have to admit that I am just speculating here.
    – user2138912
    Oct 8, 2017 at 5:16
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Here's something that may help. Bear with me. This is the 'standard voltage' that the technician should observe when checking the ecu pin for proper voltage right out of the manual. "8v or temporarily 6v or higher ⬇0-3v". Good lord, anyway my point being, fuel systems electronics are so complex. The camshaft sensor being responsible for producing the criteria needed for firing injectors. Service manual for my car has a troubleshooting guide for the fuel system that's over 200 pages long. I think that you're intelligent enough to benefit from working through the appropriate section of your vehicle's FSM and nailing down the issue the tedious but proven way. I think your wave form is fine (the picture shows one similar). One is longer to differentiate TDC of piston 1 and partner and TDC it the remaining two pistons. enter image description here

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  • You created this answer with only 8% battery left, what a hero!
    – Supertecnoboff
    Oct 12, 2017 at 16:33
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Some folks on Electronics Stack Exchange gave me hints how to wire up that sensor in lab environment outside the car and this is how the amplitude and edges are supposed to look like:

enter image description here

Also, I had to intercept between Black and Black/Blue wire. All this time I was incorrectly intercepting between Black and Pink wire because I saw some signal there and incorrectly assumed that Pink wire gotta be the sensor's output. I think the pink wire was fluctuating because car had some power supply issues to the sensor and when sensor reacted it consumed more power and pulled down the power supply wire.

While this test does not "prove" anything about spacing between pulses coming from the sensor, because I did not use reluctor ring, but rather simply moved wrench in front of the sensor, I think it does prove that at least the edges of pulses should have been steep and in range 0-5V.

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