P1570, U0100, Power Supply & Fuel Control Problems Explained
Modern vehicles rarely “just break”.
If an engine turns over but refuses to start — especially after battery failure, jump starting, water ingress, accident repair or ECU replacement — the cause is usually electronic authorisation, not a mechanical engine fault.
This guide explains why vehicles suddenly stop starting, what diagnostic codes actually mean, and how ECU internal damage, immobiliser mismatch and power supply faults disable fuelling.
First: Understand What the ECU Actually Controls
The engine ECU does not simply inject fuel.
It decides whether the engine is allowed to run.
Before injection occurs the ECU must receive:
• Immobiliser authorisation
• Valid CAN network communication
• Internal voltage stability (5V reference & 12V rails)
• Sensor plausibility checks
• Configuration verification with other modules
If any of these fail — the engine will crank forever but never fire.
Type 1 — Immobiliser Blocking Start
Symptoms
• Cranks normally but never fires
• Starts for 1–2 seconds then cuts
• “Immobiliser Active” message
• No injector pulse
• No fuel request
Common Fault Codes
P1570 — Engine Control Unit Blocked
P0513 — Incorrect Key
B2799 — Immobiliser Communication Fault
Control Module Not Authorised
Start Disabled by Immobiliser
What’s Happening
The replacement ECU or corrupted ECU identity no longer matches the vehicle’s immobiliser network (cluster, BCM, keys).
The car is working correctly — it is preventing theft.
Type 2 — No Communication With ECU
Symptoms
• Diagnostic scanner cannot connect to engine ECU
• Cooling fans running at full speed
• Multiple warning lights simultaneously
• Fuel pump behaves abnormally
• Vehicle suddenly died while driving
Typical Codes Stored In Other Modules
U0100 — Lost Communication With ECM/PCM
U0101 — Lost Communication With TCM
U0121 — Lost Communication With ABS
P0600 — Serial Communication Link Malfunction
What’s Happening
The ECU hardware has failed electrically.
However the immobiliser data inside it is still required for the car to start.
Replacing it introduces a second problem — identity mismatch.
Type 3 — Internal ECU Power Supply Failure
This is extremely common after jump starts, flat batteries or alternator faults.
Symptoms
• Cranks but injectors never fire
• Rail pressure present but no start
• Runs briefly on brake cleaner
• Random multiple sensor codes
Common Power Fault Codes
P1602 — Power Supply B+ Low Voltage
P068A — ECM Relay De-energised Too Early
P2509 — ECM Power Input Signal Intermittent
P2146 / P2149 — Injector Voltage Circuit Fault
P0606 — ECU Processor Fault
The Technical Cause
Inside the ECU are regulated voltage rails:
• 12V supply stage
• 5V sensor reference regulator
• Injector driver stage
If the 5V rail collapses, sensors report impossible values.
The ECU blocks injection to prevent engine damage.
Type 4 — Water Damaged ECU
Water ingress is one of the highest search problems and most misunderstood.
Common Entry Points
• Blocked scuttle drains
• Windscreen leaks
• Battery tray corrosion
• Accident repairs
• Engine bay pressure washing
What Happens Electrically
Water causes:
• Short circuits between tracks
• Corrosion increasing resistance
• Voltage drop on reference circuits
• CAN communication collapse
The ECU may partially function but disable fuelling for safety.
Typical results:
• Non-start
• Random CAN faults
• Multiple module communication errors
• Intermittent starting
Why Replacing The ECU Makes The Car Worse
Many owners install a used ECU expecting an instant fix.
Instead:
The engine still does not start.
Modern vehicles store identity across multiple modules:
• ECU
• Dashboard
• BCM
• Keys
• Gearbox module (often)
The replacement ECU contains another vehicle’s security data.
The network rejects it.
The Correct Repair Method — Identity Data Transfer
Instead of programming the car to accept the ECU, the correct approach is to move the original vehicle identity into the replacement unit.
This restores:
• Immobiliser synchronisation
• VIN matching
• Configuration coding
• Start authorisation
The vehicle then sees the module as factory original.
👉 Full process explained here:
https://precisionremapsuk.com/pages/ecu-tcu-cloning
Real-World Situations This Fixes
After water damage
After jump start or voltage spike
After accident repair
After ECU replacement but still won’t start
After gearbox/mechatronic replacement
After failed software update
After intermittent cutting out then total failure
When It Is NOT an ECU Problem
Mechanical failures usually show different behaviour:
No fuel pressure → fuel system issue
Uneven cranking → compression problem
Single cylinder misfire → engine fault
Starts with starting fluid → injection disabled electronically
If the vehicle previously ran and now shows immobiliser or communication faults, the cause is almost always electronic authorisation.
Key Takeaway
A non-start after ECU failure is rarely caused by the engine itself.
The vehicle is preventing operation because the control modules no longer trust each other.
Restoring the original identity data typically returns normal starting immediately.