Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I've been chasing this one for over a month. After watching some of the videos on 5v Ref circuit, I was able to finally read a wiring diagram. LOL.
The problem I have is that:
1. All of the components I check say 5.04v when unplugged and no change when unplugging the others. This seems like they are all fine.
2. O2 sensors will quickly run up to 700-800 and it goes into OL Fault and starts running very rich.
3. Before OL - Fault, It's pulling ~24% out on STB1 and STB2.
4. When I pull the Vac off the Fuel Rail Sensor, the O2 Sensors do not go back down. They stay super rich.
I've tried to lean the tables out a bit, but this seems to be more mechanical in spirit. Any advice would be very appreciated.
Forgot to mention that I did get a brief DTC about the fuel rail pressure sensor having a 5v Ref High. Even though the 5v Ref Low is still present in the DTCs as well and on the multimeter the FPS shows 5.04v.
Update: Thanks to Danner, I found that the O2 sensors were actually bad after watching them for a few min. I replaced them with some new NTK sensors and the car is finally RUNNING!!! Since the 5V wasn't the cause for this, I'm still needing to find that Gremlin.
Yep, sorry for the "Squirrel" moment. I still have this nagging 5v Ref in the DTCs. I've checked every connector that should be on that circuit and all show 5v when unplugged including the fuel rail pressure sensor, but I still have the P0642 (Sensor Reference Voltage A Circuit Low), and a P0191 (Fuel Rail Pressure Sensor A Circuit Range/Perfomance). I've changed the sensor, but code persists.
Oh, ok. I understand, sorry for the confusion on my end.
Does the p0642 code come back immediately after clearing, or do you need to use the car before it comes back? As you know, virtually any 5v sensor on the car could pull down the ref circuit as could a short to ground somewhere in the harness. Given the fuel rail pressure sensor code, I might focus my initial efforts anywhere the harness contacts the engine in the area of the FRP sensor, but don't neglect the rest of the harness. I have seen Fords with 5v ref codes and acceleratior pedal codes and the short to ground was near the fuel tank, about as far from the gas pedal as you can get.
That's interesting, I had some stuff on my plate at work to work through, but I have been thinking about this one. I would isolate the 5v ref feeds from the PCM, and if they're always 5v, I would check powers and grounds and consider changing the PCM
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