Service engine light

[matt1]

The service engine light came on the other day on my 2004 silverado. I filled up with gas on my lunch break then drove back to work (about 2 miles). When I went to leave work at the end of the day the service engine light came on as soon as I started my truck. I got out and tightened the gas cap, it felt like it may have been a little loose. I shut the truck off and restarted it to see if that was the problem but the light stayed on. I drove home (about 20 miles) and unhooked the battery for a half hour to reset it. The light still came on right away. I drove to work and back again yesterday and then unhooked the battery again and left it overnight. The light still came on right away when I started my truck this morning. Any ideas what it could be? The truck still runs great. The mods I have done to my truck include exhaust, K&N intake and Nelson tune. All of which have several thousand miles on them and have never given any trouble. Thanks for your help!

[GM-Tech]

If it's an evap code because of the loose gas cap, it won't clear until the test has run AND passed for three consecutive key cycles. Very specific conditions have to exist for the evap test to run. Just starting the vehicle up and driving it isn't a guarantee the test will run. As a result, it can take many drive cycles before the light will shut off on its own.

 

Your best bet is to get it checked to make sure that's what the problem is (was) and get the DTC cleared with a scanner.

[DJinCO]

Push the On-Star button and ask them to do a diagnostic check. You do have On-Star, don't you?

[matt1]

Can Auto Zone identify and clear the code with a scanner? I read somewhere that they will do this for free.

[budhayes3]

They should be able to get a code out of it, but since you're still covered under warranty, you should probably bring it in and get it checked out.

[BishopZ71]

I took mine to auto zone and they read mine. It said small emissions leak. d**n gas cap was broke. You could take the gas cap and turn it several times with your hand. Took it to a friend of a friend and he got the small evap code. Which was the faulty gas cap.

[matt1]

I took it to Auto Zone to have scanned and the code P0315 came up. The scanner read crankshaft postion sensor learn fault or something like that. I went ahead and purchased the scanner as I have been wanting one for a long time. When I got home I tried to erase the code but it will not erase. I don't understand how there could be something wrong with the CPS since my truck runs fine. I would think the timing would be off and it wouldn't run right. Anybody have any ideas how to fix this? Where on the motor is the CPS located? Thanks.

[GM-Tech]

I took it to Auto Zone to have scanned and the code P0315 came up.

 

The crankshaft position learn is something that is done with a Tech 2 scanner. That's the scanner we use in service. I doubt your scanner can do anything more than display a DTC.

 

This code usually only sets on a reprogramming event, but there are other things that can cause it. A simple cause is disconnecting the battery with the key ON.

 

The learn procedure takes about 30 seconds to do but can only be done with a scanner that has that capability. If a relearn doesn't eliminate the problem (unlikely) you have bigger problems.

 

Here's what it's all about...

 

The crankshaft position (CKP) system variation learn feature is used to calculate reference period errors caused by slight tolerance variations in the crankshaft, and the CKP sensor. The calculated error allows the powertrain control module (PCM) to accurately compensate for reference period variations. This enhances the ability of the PCM to detect misfire events over a wide range of engine speed and load. The PCM stores the Crankshaft Position System Variation values after a learn procedure has been performed. If the actual crankshaft position variation is not within the Crankshaft Position System Variation compensating values stored in the PCM, DTC P0300 may set. If the CKP system variation values are not stored in the PCM memory, DTC P0315 sets.

 

[budhayes3]

Did this code pop up after installing the tuned PCM? I know that sometimes new PCM's require a crank relearn that can only be done with a TechII or equivalent scanner. I think this is what GM-Tech is referring to.

[matt1]

I've got around two thousand miles on my new nelson tuned pcm. I never had any problems at all after installing it until now. I've heard of people having to do a crankshaft position senor relearn when installing a new tuned pcm but I thought that that was something that had to be done right away. My truck runs and starts great. I know its just got to be something simple.I hadn't unhooked my battery at all since I installed the new pcm. So I don't see how unhooking the battery with the key on could have done it.

[budhayes3]

Where on the motor is the CPS located? Thanks. 

 

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The crankshaft position sensor is on the passenger's side of the engine block, hidden behind the starter. If you remove the starter you will see a 3 wire connector going to the sensor that's held in with 1 bolt...don't remember what size.