No start

[99WhiteC5Coupe]

I checked the run/start relay via multimeter. Shows it’s operating normal. I’m seriously at a loss.

Have you cleaned the battery cable connectors with a battery brush tool and switched to a known good battery?

 

[MTX65]

Second what @99WhiteC5Coupe said.


You can not diagnose anything electrical until you know it has and is getting the correct voltage.
Start with your battery Go get it tested properly, any Auto parts stores do it for free.

Once you know that you have a good battery
do a Voltage Drop Test to make sure all your connections and cables are good.

Only then can you start to diagnose. Unless you like running in circles

Had the battery checked (voltage and load tested) and it all came back normal. I’ll have to look into drop testing.

 

[MTX65]

Have you cleaned the battery cable connectors with a battery brush tool and switched to a known good battery?

Connectors and posts were cleaned thoroughly. Haven’t switched to a known good battery but I did have it tested and it came back fine. Still a chance that a new battery could do the trick?

 

[Yupster Dog]

Check ground connections.
A lifted ground can cause many systems to mess up at the same time like that.

 

[MTX65]

Check ground connections.
A lifted ground can cause many systems to mess up at the same time like that.

I checked all the grounds in the engine compartment. Haven’t checked the ones in the cab yet. Got a new battery. The other day the truck would start right up if I skipped the on position. Just put the key in and turn all the way and it would crank. While running I would get various random codes or no codes at all. (Abs module codes, IC codes, RCM codes) but they were never consistent and would change. Fast forward to today I put a new battery in it and made another attempt. Truck wouldn’t start. I initially tried starting it as usual (on position let the dash cycle and then start) unsuccessfully, then tried to trick it by skipping the on position again and even that didn’t work this time. (I’m assuming because by making the first attempt the truck is storing whatever information it needs to keep it from starting after shutting off.)

 

[Yupster Dog]

(I’m assuming because

Found the problem. you are running in circles instead of actual diagnostics to rule out and move forward.

Nobody can help you from across the internet on your assumptions.

Start over with a "Voltage Drop Test" of all the battery cables (positive and negative)
Check the ground coming off the PCM And other grounds under dash.

You need to finish a diagnostic to rule it out.