2004 GMC Envoy Knock Sensor

SAMY1969

MEMBER
2004 GMC ENVOY
V8
2WD
AUTOMATIC
81,000 MILES

I took in my car for an inspection and it failed. The inspector said my knock sensor shows a low, or no, voltage reading, and when this happens the emissions test cannot run. The engine has always run fine, and my gas mileage has been consistent (380-400 miles per tank for 5 years now). I took the vehicle to a mechanic, he plugged in his computer equipment to check the error codes, same thing. He said the sensor is inside the engine, meaning he'd have to remove to intake manifold. Cost to repair is $800-$1000. Ugh.

I bought a Chilton's manual to read up on this and he was right, it appears the sensor for my year-2004 5.3L V-8 is in fact inside the engine. I saw in the manual there are 2 sensors. If the engine continues to run at the same level it did when I drove it off the lot, why does the sensor matter? Also, is there a way to bypass the sensor so that the error code doesn't get thrown when the system is tested?

Sam

3 Replies

MERLIN2021

MECHANIC
17,250 POSTS

No the sensor cant be bypassed, and it matters because the PCM cant clear and if codes are pressent the emmosiion readyiness state show the truck is not ready! Must replace the sensor, you may in fact improve the mileage by changing the sensor, or repairing the wires. Knock sensors will retard the timing, using more gas!

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Sunday, March 15th, 2009 AT 2:05 PM

SAMY1969

MEMBER
2 POSTS

Thanks for the quick reply merlin2021. I am interested in the comment about the wires. Are the wires in a closed-loop system? If so, why can't I just cut them from the engine block and connect them together to complete the circuit?

Assuming I can't just complete the loop, I will ask my mechanic to check the wiring first (though it's doubtful that's the problem). I haven't had any problems with this car until now, and I don't believe my mileage will increase by repairing the sensor (it's always given me 17.6 mpg, and the sticker said 17 hwy).

I'm also worried about the performance after the engine is ripped apart. What sorts of things should I check before taking delivery of the repaired vehicle? I don't have a compression tester, nor anything that could check for vaccuum leaks. I guess I'm just worried that something will go wrong, and the mechanic will claim it wasn't his fault. So I'd like to be prepared with whatever a consumer should be prepared for (asking the right questions, taking pictures of the engine beforehand, etc.). Please help.

Sam

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Sunday, March 15th, 2009 AT 9:22 PM

MERLIN2021

MECHANIC
17,250 POSTS

You know what Sam if you want to experiment with it leave enough wire so it can be re connected, not sure what that will do to the voltage signal. That sensor operates in both open and closed loop.

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Monday, March 16th, 2009 AT 1:33 PM