Check engine light on
 

I have a 2006 5.3 Silverado with 154000+ miles, about a month ago my check engine light came on and according to the mechanic it was the knock sensor so he recommended replacing both, which he did. Now my check engine light came on again. Anyone have any suggestions? The mechanic says the reader shows knock sensors again. Can it be something else? I haven't got a diagnostic check done.

What is the code that you are getting. Autozone and Advance Auto parts will put their scanner on and read the code for you for free.

I didn't get that info from the mechanic. I asked him and he said it could be a couple of things , sensors or even the computer module. he suggested a diagnostic, which I have already set up an appointment for next week. I am curios if anyone else has gone through this process of a mechanic just saying it was knock sensors and then being something else.

I just had a diagnostic test done. Dealership says its the knock sensor harness because it's moving and giving different ohms readings, and the pcm has bad modules. Total cost to repair is 1500 for new oem knock sensors and harness as well as a new pcm. It seems pretty steep since the total of all parts is only 500. Any suggestions?

ks harness replaced
 

Well I just replaced my knock sensor harness and rechecked the knock sensors and they checked out fine, Now after driving the truck for one day the check engine light has returned, Anyone have any suggestions on replacing my pcm, or do I just need to take it to the dealership and have them do it?

Hi mibar,

I'm sorry to hear that your Check Engine Light has returned. I understand that you are seeking opinions from the other forum members in regard to your concern. If you decide to go the dealership route feel free to let us know. If needed, we can contact your dealer on your behalf to discuss your concern with them. We'd be happy to help!

Kristen A.
Chevrolet Customer Care

if the pcm has bad modules it needs to be replaced and reprogrammed by the dealer. you could find a pcm online and install it then tow it to a dealer to have it programed

I have found an auto engineering mechanic in town that says he will program it for 125.00. I haven't got a quote from the dealer yet.

Not trying to be a dick here but, have any of these mechanics checked for a legitimate reason for that sensor to register, as in a legitimate knock issue caused by incorrect timing, poor fuel quality, compression fault or any one of the various things the computer says is incorrect information coming from it.

I'm sorry, I am much quicker to believe that there is an issue within the motor it self after a hundred and fifty thousand miles than a PCM failure, (BTW, I have experienced PCM failure).

I agree, that is a very valid point, it could very well be detecting a knock...
might try a different grade of fuel and getting it from a different station as an easy place to start.

I will try switching to a different grade and fuel. As far as timing and a miss of some type. The truck just passed a smog inspection here in California in September and everything was just fine, no problem and it happened just after the knock sensor was replaced, the engine light had not come back on.

Judging by your original post id sounds like it should not have. I do not know specifics about CA inspection, only that they are similar to but more strict than NY.

Your average vehicle owner does not know the nuances of the diagnostic system and its tools. That said here is a quick rundown of the important stuff. With so many moving parts packed into such a small space one thing goes wrong it is going to have impact on many other parts. So there are sensors designed to...well, sense a malfunction at key points. When something goes wrong they relay to the CPU a generic message that there is a malfunction. The CPU stores then relays that to the owner or mechanic through the OBD system, This much everyone knows.

What many people don't know is there are three stages of code. There is monitor, pending and mature.

Monitor: A system initialization so to speak, Also known as readiness check. (CEL is off during this process.) When codes are manually cleared via diagnostic tool, CPU battery depletion or (in some but not all cases) sensor change it goes. Again it is as it sounds, the CPU is monitoring the sensors to gather the required information because for whatever reason it no longer has that information. Monitors have various requirements needed to clear them, most of them involve driving time.

Pending: A system double check so to speak, an enhanced monitor. (CEL is off during this process.) Basically the monitor clearance requirements have been meet but the information the CPU is receiving has changed. One of two things is happening because the either one, the CPU knows there was an active code but the information has changed indicating possible repair so it is double checking that information to determine if it has been fixed or two, the information change indicates a malfunction and the CPU is double checking to confirm that there is infact a malfunction. Pending status is like I said an enhanced monitor If memory serves me correctly, to clear a pending status the owner needs to meet the requirements to clear the monitor again.

Mature: An active malfunction code. (CEL is on during this process.) Basically all monitors have cleared and Pending codes have been confirmed and there is a malfunction.


Now, going back to what started this spchiel, In NY an OBD inspection is failed or can not be completed for any one of these statuses being active. It sounds to me like your inspection station failed to see either an active monitor or a pending code. While yes, sensors do periodically fail more often than not it is not a sensor failure that triggers the light. By all rights you should have had a pending code on that knock sensor before you got it inspected.

But anyway, Fuel is a good place to start like in2pro recommended, but if that does not cure the issue do some research on the internal causes that can trigger the knock sensor. With 150000+ you are getting into the ware and tare age of the truck and some internal parts that have been moving and rubbing together are going to be worn and there may be something just worn out. This is far more likely and plausible than a CPU failure.

When your CPU fails, I kid you not your truck will literally become stupid and won't know the difference between a starter and a vacuum line. Trust me, been there. Startup gets retarded, CEL comes on, goes off randomly, sometimes it flat refuses to start, so much crap all at once your head will spin, at least this was the case with my 98 GMC 1500.

[QUOTE=Ronin1014;287069]Judging by your original post id sounds like it should not have. I do not know specifics about CA inspection, only that they are similar to but more strict than NY.

Your average vehicle owner does not know the nuances of the diagnostic system and its tools. That said here is a quick rundown of the important stuff. With so many moving parts packed into such a small space one thing goes wrong it is going to have impact on many other parts. So there are sensors designed to...well, sense a malfunction at key points.[QUOTE]

Thanks for your information. I truly did not understand how the whole concept works, but with your info I feel I have a better understanding. We will try the gas change and we will go from there. Truly appreciate it.

Not a problem, I watched my brother in law grow completely disenchanted with his 98 Reg cab short box Silverado after replacing the same 02 sensor five or six times and having it come right back. I didn't know all that then and he pretty much tossed nice truck.

you make a very good point but there are a few codes for knock sensor issues being circuit failure and or things you described. it usually will throw a code for cam crank correlation and also have a p0300 for bad fuel quality but the codes for knock sensor directly meaning circuit failure or sensor itself ive had the harness ground out and fry the pcm modules. its highly possible if sensors unplugged and engine runs great while code persists its usually a sensor/circuit failure.

Since the harness was detected to be loose during the diagnostic test and after both sensors had been replaced and the dealer suggested a new pcm and the codes its throwing are only sensor failure codes maybe their is a fried pcm modules. I'm just trying to make sure it truly needs a pcm before I spend all that money. Don't get me wrong I don't mind spending the money, I love my truck and have always maintained it well, but I don't want to be disappointed by not being the pcm. I really appreciate all the input. Thanks.