On my wife’s 2019 F-150 (3.5 EB) I all of a sudden got all of these warnings:
- Park break limited function. Service required
- See manual
- Service AdvanceTrac
- Hill Descent Control Fault
- Park break limited function. Service required
- Hill Start Assist not available
- Pre-Collision Assist not available
The truck wouldn’t even crank. I changed the PCM relay and the messages went away. Now the truck cranks but doesn’t start. Fuel pump fuse is good. The messages randomly come back and then go away again. The truck also has a brand new battery that is fully charged. Any ideas? Thank you very much

 

Throttle body is often the problem with those codes.

 

Can bus error is common. Only way to really know without loading the parts cannon is to take it to a diagnostic clinic

 

Scan for codes and tell us what they are. Otherwise, we are just guessing.

 

Thank you for the feedback.

The codes I got from Forscan were these:U0121 - Lost Communication With AntiLock Brake System (ABS) Control Module 'A'
Parking Aid Module

U0100 - Lost Communication With ECM/ PCM A
Parking Aid Module and same on Telematic Control Unit Module



C1D00 - Park Brake Apply Switch

Additional Fault Symptom:

- Circuit Short To Ground


B143B - Autolamp On Input

Additional Fault Symptom:

- Circuit Short To Ground


B143C - Headlamp Off Input

Additional Fault Symptom:

- Circuit Short To Ground


Transfer case module:
U0101:00-08 Lost Communication Wit…

Antilock breaking system:

P05D3:86-28 Driver Mode Select Swit...

U0100:00-68 Lost Communication Wit...

I rebooted the modules with Forscan and the codes did not come back.

Thank you so much for your help

 

Anybody has any ideas? I’m still struggling with this and would like to get the truck going again and avoid the stealership… Thank you

 

Fire it up and hit the hazards button. Walk around and make sure are your bulbs are functioning properly. Check that the 3rd brake light on top of the back of the cab is also working properly.

 

All the light bulbs work great. Also got a new throttle body and put that in and it didn’t change anything…

Any other ideas?

Thank you so much for your help

 

.
Fault codes cannot be >fixed< by clearing. The faults are still there.
The two B codes are referring to the same fault. Some feature involving a hands-free mike use.
C 1D00 Park Brake wire fault. left side of Dash down lower
P 05D3 is shift mode select, likely a wire issue. Switch Button is in the end of the Shifter unless you have Floor shifter.
.
The lost comms can be ignored and would clear when all the faults are cleared.
.
I was just looking at my 2018 Pids today, and tested all the shift modes selections along with all the others the vehicle has, using my high-end Scanner.
Nothing like having aa way to test and see all these functions.
.
Your codes are for all different circuits.
My guess is a Rodent got in the engine Bay or Cab at some harness to cause these unrelated issues, as a common point area.
..
Good luck.

 

Critical Observations

1️⃣ U0100 + U0121 Together

If PCM and ABS both drop, that points to:

• HS-CAN bus fault
  
• Common power feed issue
  
• Shared ground issue
  
• Smart Junction Box (BCM) or underhood fuse box problem
  

Not a single module failure.

2️⃣ Multiple “Circuit Short To Ground” Codes (B143B, B143C, C1D00)

When unrelated inputs show short-to-ground simultaneously, that usually means:

• Reference voltage collapse
  
• Ground reference shift
  
• Module internal reset due to undervoltage
  

These are rarely real switch shorts when they all appear together.

3️⃣ No Start After Relay Swap

Now you have crank/no start.

That narrows it:

• PCM may not be powering injectors
  
• Fuel pump driver module may not be receiving command
  
• PCM losing power during crank
  
• CAN bus not initializing
  

If PCM drops during crank → injectors disabled → no start.

High Probability Root Causes (Ranked)

1️⃣ Failing Underhood Fuse Box (BJB)

Common on F150s.

Internal bussing cracks cause:

• Intermittent PCM power loss
  
• Random CAN faults
  
• False short-to-ground codes
  
• Crank/no start
  

Very high probability.

2️⃣ Poor Main Engine Ground

Check:

• Battery negative to frame
  
• Frame to engine
  
• Engine to body
  

Load test voltage drop during crank.

You should see:

Battery voltage during crank ≥ 10.5V
Voltage drop battery neg to engine block < 0.2V

Anything higher → ground issue.

3️⃣ PCM Power Feed Instability

Check with wiring diagram:

• PCM constant B+
  
• PCM switched ignition feed
  
• PCM grounds
  

Backprobe and monitor voltage while cranking.

If it drops below ~9.5V → PCM resets → no start.

4️⃣ HS-CAN Bus Short

Less likely but possible.

Measure resistance between CAN+ and CAN– at DLC:

Ignition off:
Should read ~60 ohms.

If:

• 120 ohms → one termination missing
  
• 0–30 ohms → short
  
• fluctuating → network instability
  

5️⃣ Fuel Pump Driver Module (FPDM)

Check if pump primes key-on.

If no prime:

• Verify PCM is commanding pump
  
• Check FPDM power and ground
  

But given your network symptoms, FPDM failure alone is unlikely.

What I Would Do Next (In Order)

Step 1: Voltage Drop Testing Under Load

Not static battery testing. Under crank load.

Measure:

• Battery positive to PCM B+
  
• Battery negative to engine block
  
• Battery negative to body
  
• Battery positive to underhood fuse box output
  

If you see >0.5V drop on power side or >0.2V on ground side → found issue.

Step 2: Inspect Underhood Fuse Box (BJB)

Pull it.
Check:

• Melted terminals
  
• Loose PCM relay cavity
  
• Heat discoloration
  
• Corrosion under fuse box
  

This is a known weak point.

Step 3: Monitor PCM PID in Forscan During Crank

Watch:

• RPM
  
• Injector pulse width
  
• Fuel rail pressure
  
• PCM voltage
  

If:

• No RPM → crank sensor
  
• No injector pulse → PCM disabled
  
• PCM voltage drops during crank → power issue
  

Step 4: Check CAN Resistance at DLC

Pin 6 to 14.
Ignition off.
Should be 60 ohms.

Why Rebooting Modules “Fixed” It

Module reset cleared transient undervoltage flags.

It does NOT fix the root cause.

If it was a true ABS or PCM failure, codes would immediately return.

The fact they didn’t supports:
→ Intermittent power/network issue.

Important Failure Pattern

When:

• AdvanceTrac
  
• ABS
  
• Pre-Collision
  
• Hill Start
  
• Parking Brake
  
• Transfer Case
  

ALL throw faults at once

That is almost always:

Power distribution or CAN integrity failure.

What This Is NOT

• Not just a bad fuel pump
  
• Not just a bad ABS module
  
• Not just a bad battery
  
• Not random coincidence
  
• Not a software issue
  

Production-Grade Assessment

What’s Missing

• Voltage drop measurements under crank
  
• CAN resistance measurement
  
• PCM voltage during crank
  
• Fuel rail pressure data
  
• Confirmation of pump prime
  

What’s Required for Proper Diagnosis

• Wiring diagram access
  
• Load testing power and grounds
  
• Oscilloscope for CAN (ideal but optional)
  
• Backprobe testing under live crank conditions
  

Likely Repair

Most probable:

• Replace underhood fuse box (BJB)
  OR
  
• Repair main ground / power feed issue