my truck started running really rough earlier today and the cel came on for a moment when it happened. i checked the codes and this is what it threw:

181- (trucks only) HEGO (HO2S) sensor fault unable to switch/lean during part throttle
334-EGR closed valve voltage high
556- fuel pump relay primary circuit fault

the o2 sensor was replaced about a year or two ago. also, sometimes when i start it after its been sitting, it will die and then i have to start it again. which probably has something to do with code 556. where is the fuel pump relay? the truck is running better now, it seemed to clear up as i drove.

 

The FP relay should be located under the hood on the driver side, near the firewall. About in the same area as your diagnostics plug, but which one it is I can't say for sure.

I've been sorting out a code 334 on my 1991 5.8 lately. What you'll want to do is backprobe the harness for the EVP sensor and see what it's reading at idle. Then unplug the vac supply and see what happens. If the voltage stays the same, either the EGR is stuck open or the sensor is bad. If it drops significantly, your EVR (EGR vacuum regulator [also called a solenoid]) is likely bad. Fuel Injection Technical Library » EGR Vacuum Regulator (EVR) In my case, I had .92 volts at idle, and it dropped to .45 volts when I removed vacuum, so my regulator was bad.

Good luck.

 

Fuel Pump Relay location info:

courtesy of subford

 

I've got the code 556-Primary Circuit issue code also..

What does it ACTUALLY mean? I mean, which wires/circuit is having issues? And what can those issues be?

Right now, I'm getting the codes for the engine running lean (Has a vacuum leak upstream of the Oxygen Sensor, which I'm sure is not helping the case) and every so often, I can smell 1/2 burnt fuel from the exhaust.

But otherwise I'm still getting "Decent" mpg..

 

my problem is getting much worse. when i start the truck in the morning in dies and/or runs rough for a few minutes. it seems to clear up once its warm. il have to start checking voltages. the egr vaccuume solenoid and egr sensor have been replaced within the past year. i think the egr valve is getting hung up. it looks original. what symptoms would a bad fuel pump relay cause?