I have a 04 chevy avalanche I keep getting flashing engine light when I run above 60 usually down hill misfire code p0300 I have done the spark plugs wires and coil packs. I did the cam sensor and crankshaft sensor that's when it started throwing codes. I added a couple ground wire to the tps sensor and a couple other spots which where a problem before I did the sensors. If anyone knows what the problem can be let me know. And there is no water in the gas and have good fuel pressure

 

checkfor clogged cats....remove the front 02 sensors from the exhaust...this allows the exhaust stream to bypass the cats.

 

What would removing the o2 sensors do? And wouldn't that bring a code up for the o2 sensors?

 

iif the cats are clogged...the exhaust can't escape out the tailpipe....no air out equals no air in...the engine is a air pump. unscrewing the o2 sensors from the exhaust allows a place for the exhaust to exit.
yes 02 sensor codes will set but if the engine runs smooth and the flashing cel/ misfires are gone...you know what the problem is.


if a mechanic want to test for restricted exhaust...he/she would remove the 02 sensor and screw a pressure gauge into the hole to measure the amount of backpressure. Since you don't have a gauge...you test it by driving it.

 

Thank you for the information will do as soon as possible.

 

I got it fixed by having a crankshaft sensor relearn mechanics will say it don't call for it but a dealership will do it for under 100 bucks

 

its required when a pcm or crankshaft position sensor is replaced. typically it causes false misfire codes but no driveability problems.