94 K1500 check engine light no codes

[82corvette]

I bought my K1500 new in Oct 93 and its been a great truck. About a year ago the cel started coming on & I could not retrieve any codes. I have replaced the fuel pump with a GM Delphi unit along with a fuel filter, cap, rotor,wires,plugs, coolant temp sensor,iac,egr valve, knock sensor, map sensor, throttle body gasket and cleaned the throttle body. Today I installed a reman ecm & no change. This was an occasional light that seems to be getting worse. Now it will come on at start up at times but sometimes waits till I drive a few miles. Now it will go into failsafe part of the time. Usually it runs great. It has 168,000 mostly highway miles and uses no oil. It has had the best in care & never been abused. What is ailing this truck?

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[Schurkey]

1994 model year?

No codes, or no codes you can retrieve with a paper-clip?

Connect a REAL scan tool, even if there aren't codes, you may be able to spot anomalies in the data stream.

 

[ram1220]

O2 sensor? My 90 started setting the CEL years ago only at highway speeds and only after about 5 minutes on the highway. I changed out the O2 sensor and the EGR valve. Problem solved. I noticed the O2 sensor was not listed in the parts you have changed.

 

[Schurkey]

I had a similar experience with an O2 sensor setting the Check Engine Light at highway speed. Two differences: it set a code, and it never went into Limp Home mode.

 

[82corvette]

Finally! I was looking around underneath and noticed the knock sensor had been damaged and the plastic connector was loose in the housing so I replaced it. The truck quit going into limp mode but the CEL still stayed on. I was able to retrieve 2 codes #32 & #22. I replaced the egr solenoid & the throttle position sensor, (for the second time!) and have driven it over 500 miles without a CEL or any problems. Just goes to show even new parts fail. Thanks everyone for helping!