Seeing C1310 and U0100 on a Toyota Prius can be confusing because the two codes come from different systems.
C1310 is generally stored by the brake or skid-control ECU after the hybrid control system reports a malfunction. U0100 means a control module has lost communication with the Engine Control Module, or ECM, over the vehicle’s CAN communication network.
Neither code directly diagnoses a failing high-voltage hybrid battery.
A weak 12-volt battery, damaged wiring, poor power or ground, a communication-network problem, or another hybrid-system fault may contribute to the symptoms. The correct next step is to scan all relevant control modules and find the primary fault before replacing the hybrid battery or another major component.
C1310 and U0100 at a Glance
Question | Quick answer |
What does C1310 mean? | The brake control ECU received a hybrid-system malfunction signal |
What does U0100 mean? | A control module lost communication with the ECM or PCM |
Do these codes confirm hybrid battery failure? | No |
Could the 12-volt system be involved? | Yes, but it should be tested rather than assumed to be the cause |
What should you do next? | Run a complete hybrid-system scan and diagnose the primary hybrid or CAN communication fault |
Not sure whether your Prius problem involves the battery, 12-volt system, or vehicle electronics?
What Does C1310 Mean on a Prius?
C1310 is stored by the Prius brake or skid-control ECU.
Toyota describes C1310 as a malfunction in the hybrid vehicle system. The brake ECU communicates with the hybrid control ECU because regenerative braking and conventional hydraulic braking need to work together. When the hybrid control ECU reports a malfunction, the brake ECU may store C1310 and limit part of the electronically controlled braking system.
Question | Quick answer |
Which module stores C1310? | Brake or skid-control ECU |
Does it identify the failed part? | No |
Can the hybrid battery be the underlying cause? | Yes |
Can another hybrid component be responsible? | Yes |
Best next step | Scan the hybrid control system for the primary DTC |
Why C1310 Is Usually a Secondary Code
Toyota’s inspection procedure for C1310 does not direct the technician to replace a brake component. It directs diagnosis toward the hybrid control system.
The primary problem could involve:
- The high-voltage battery
- The inverter or converter
- A motor-generator
- The gasoline engine
- Hybrid-system communication
- The hybrid transaxle
- Another high-voltage or powertrain fault
C1310 tells you that the brake system knows a hybrid fault exists. It does not tell you which hybrid component caused it.
Does C1310 Mean the Hybrid Battery Is Bad?
Not by itself. C1310 may appear when a battery problem exists, but stronger evidence would come from battery-specific codes such as:
- P0A80
- P0A7F
- Applicable P3011 through P3024 weak battery-block codes
- Other model-specific battery deterioration codes
If one of those appears alongside C1310, the battery becomes a much stronger diagnostic direction.
If C1310 appears without a battery-specific code, the rest of the hybrid control system should be scanned before concluding.
The absence of P0A80 also does not prove the battery is healthy. Battery faults can involve sensors, temperature, cooling, wiring, control circuits, or other conditions that use different DTCs.
What Does U0100 Mean on a Prius?
U0100 means Lost Communication With ECM/PCM “A.”
On the Prius, the ECM communicates with the hybrid control ECU, battery ECU, and other modules over the Controller Area Network, commonly called the CAN bus.
Toyota’s Gen 2 Prius battery-system repair information defines U0100 as a CAN communication malfunction in which the battery ECU does not receive a signal from the ECM. Toyota directs technicians to diagnose the CAN communication system when the code is present.
The Prius hybrid control ECU can also store U0100 with information codes that describe different types of communication failure between the ECM and hybrid control ECU.
What Is the CAN Bus?
The CAN bus is the communication network that allows vehicle computers to exchange information.
On a Prius, those modules may include:
- Hybrid control ECU
- Engine ECM
- Battery ECU
- Brake or skid-control ECU
- Electric power steering ECU
- Gateway ECU
- Other vehicle-control modules
Toyota’s Prius network documentation lists U0100 and several other U-codes as communication faults involving these modules.
A communication failure can affect vehicle operation even when the hardware being controlled is mechanically or electrically healthy.
What Can Cause U0100?
Possible causes include:
Low or unstable 12-volt power
The 12-volt battery helps power and initialize the vehicle’s electronic control modules.
If low voltage prevents one module from starting correctly, another controller may temporarily lose communication with it.
This makes testing the auxiliary battery reasonable, especially when:
- Several communication codes appeared together
- The Prius recently had a dead 12-volt battery
- Warning lights appeared during startup
- The vehicle intermittently fails to enter READY mode
- Electrical behavior is inconsistent
However, U0100 should not automatically be blamed on the 12-volt battery.
Loss of ECM power or ground
The ECM cannot communicate if it is not properly powered.
Possible issues include:
- Blown fuse
- Poor ground
- Loose connector
- Damaged power wire
- Corrosion
- Previous electrical service
CAN wiring problem
Toyota identifies the CAN communication system as the primary diagnostic area for U0100 on the Prius.
Network problems may include:
- Open CAN wiring
- Shorted CAN wiring
- Damaged connectors
- Corroded terminals
- Water intrusion
- Harness damage
- Improper aftermarket wiring
Control-module problem
An ECM or another module can fail internally, but module replacement should normally follow verification of:
- Power
- Ground
- Fuses
- Wiring
- Connectors
- CAN network integrity
A lost-communication code identifies the module that cannot be heard. It does not automatically prove that the module itself is defective.
How C1310 and U0100 Can Appear Together
These codes can be related because both the brake and hybrid systems depend on network communication.
One possible sequence is:
- Communication with the ECM is interrupted.
- The hybrid control ECU recognizes that important engine information is unavailable.
- Hybrid-system operation is limited.
- The brake ECU receives notification that the hybrid system has a malfunction.
- C1310 is stored as a secondary brake-control code.
That is one possible interpretation, not a universal rule.
Toyota specifically states that when CAN communication DTCs occur alongside other codes, the communication problem should be corrected first before troubleshooting the other DTCs.
This makes U0100 particularly important when it is current and repeatedly returns.
Does C1310 + U0100 Mean the 12-Volt Battery Is Bad?
No. A weak 12-volt battery can contribute to communication problems, but these two codes do not prove that the auxiliary battery is the root cause.
The 12-volt battery should be tested when:
- Startup voltage is unstable
- The battery has recently discharged
- Several modules have low-voltage or communication codes
- The vehicle intermittently enters READY mode
- Electrical problems occur primarily during startup
If the 12-volt battery tests healthy, diagnosis should continue through:
- ECM power and ground
- CAN wiring
- Connectors
- Fuses
- Hybrid control communication
- Other stored DTCs
The Hybrid Geek can test the 12-volt system during a mobile diagnostic and provide a compatible AGM replacement when the battery itself fails testing.
Does U0100 Mean the Hybrid Battery Is Fine?
No. U0100 is not evidence for or against high-voltage battery health. It tells you that communication with the ECM was interrupted.
The Prius could theoretically have:
- Only a communication problem
- Only a hybrid battery problem
- Both problems at the same time
That is why all modules should be scanned before deciding whether the traction battery is involved.
What Battery Codes Should You Look For?
If hybrid battery deterioration is suspected, look for more direct battery codes such as:
P0A80: Replace Hybrid Battery Pack
P0A80 is one of the strongest indicators that the high-voltage battery has developed unacceptable performance or imbalance.
P0A7F: Hybrid Battery Pack Deterioration
P0A7F indicates that battery performance has deteriorated.
P3011 Through P3024
On applicable Gen 2 Prius systems, these codes identify a monitored battery block that has become weak.
The presence of one of these codes changes the diagnostic picture considerably.
For example:
C1310 + U0100 only
Focus first on the communication fault and search for additional primary codes.
C1310 + P0A80
The high-voltage battery becomes a strong diagnostic direction.
C1310 + U0100 + P0A80
There may be both a communication problem and a confirmed battery-performance problem. One should not automatically be assumed to explain the other.
Common Symptoms
C1310 and U0100 may be accompanied by:
- Master warning light
- Check engine light
- ABS or brake warning
- VSC or traction warning
- Hybrid-system warning
- Reduced regenerative braking
- Reduced-power operation
- Failure to enter READY mode
- Intermittent starting
- Multiple warning lights appearing together
The exact symptoms depend on which system is causing the primary fault.
Toyota notes that communication failures can cause control modules to enter fail-safe operation.
How Serious Are These Codes?
Arrange prompt diagnosis when:
- U0100 repeatedly returns
- C1310 remains after codes are cleared
- Multiple warning lights are present
- The vehicle starts intermittently
- Regenerative braking appears reduced
- Several communication codes are stored
- The problem began after electrical service
- A battery-specific DTC is also present
Stop driving and seek assistance when:
- The Prius loses propulsion
- It cannot maintain safe road speed
- It will not enter or remain in READY mode
- Braking feels abnormal
- Steering or braking assistance changes
- The vehicle behaves unpredictably
- There is smoke, unusual heat, or a burning smell
Toyota’s electronically controlled brake system includes fail-safe operation, but that does not mean a vehicle with active brake or hybrid warnings should automatically be considered safe to drive.
How C1310 and U0100 Should Be Diagnosed
Step 1: Run a complete vehicle scan
Do not rely on an engine-only code reader.
Scan:
- Hybrid control ECU
- Battery ECU
- Engine ECM
- Brake or skid-control ECU
- Inverter-related modules
- Power steering ECU
- Gateway and communication modules
Step 2: Record the complete code set
Save:
- C1310
- U0100
- Information codes
- P-codes
- Other U-codes
- Brake-system codes
- Code status
- Freeze-frame information
Do not clear the codes until this information has been saved.
Step 3: Diagnose CAN communication faults first
Toyota directs technicians to address CAN communication problems before troubleshooting simultaneously stored non-CAN DTCs.
Diagnosis may include:
- Checking which modules respond to the scan
- Testing 12-volt battery condition
- Verifying ECM power and ground
- Checking related fuses
- Inspecting CAN wiring and connectors
- Checking network resistance and signal integrity
- Following the correct Prius wiring diagram
Step 4: Find the primary hybrid fault behind C1310
Once communication is reliable, inspect the hybrid control module for more specific codes.
C1310 should normally lead the technician back to the hybrid system rather than being treated as the root fault itself.
Step 5: Evaluate the hybrid battery only when the evidence supports it
Battery diagnosis may include:
- Battery-block voltages
- Voltage behavior under load
- Internal resistance
- Battery temperatures
- State-of-charge behavior
- Weak-block codes
- P0A80 or P0A7F
- Battery cooling performance
This separates an actual battery-performance problem from a communication problem that merely produces similar warning lights.
Choosing the Right Solution
Route 1: Repair a 12-Volt Power Problem
This may be appropriate when testing identifies:
- Weak auxiliary battery
- Loose battery terminals
- Poor ground
- Low module supply voltage
- Repeated battery discharge
The Hybrid Geek can test the auxiliary battery and provide a compatible AGM replacement when needed.
Route 2: Repair the CAN Communication Problem
This route applies when diagnosis finds:
- Damaged wiring
- Corrosion
- Loose connector
- Missing ECM power or ground
- Network short or open circuit
- Fuse problem
- Another communication-network fault
Replacing the hybrid battery will not correct an external CAN communication problem.
Route 3: Diagnose the Primary Hybrid-System Fault
C1310 should lead to the more specific DTC stored in the hybrid system.
The repair may involve:
- Engine system
- Inverter
- Hybrid transaxle
- Motor-generator
- High-voltage battery
- Another control component
Route 4: Schedule a Mobile Diagnostic
A Hybrid Geek mobile diagnostic is a strong starting point when:
- C1310 and U0100 appear together
- You are unsure whether the battery is involved
- The vehicle has several warning lights
- The 12-volt battery may be weak
- A basic scanner provided incomplete results
- You want evidence before replacing a hybrid battery
The Hybrid Geek can review the code set, test the 12-volt battery, and evaluate available high-voltage battery data.
Some advanced CAN wiring, ECM, brake, programming, or other control-module repairs may require an appropriately equipped Toyota or hybrid repair facility.
Route 5: Replace the Hybrid Battery When Failure Is Confirmed
Battery replacement becomes appropriate when separate testing identifies:
- P0A80
- P0A7F
- Weak battery-block codes
- Repeatable block imbalance
- Significant capacity loss
- Other confirmed high-voltage battery deterioration
When battery failure is confirmed, The Hybrid Geek can help compare replacement battery options and arrange mobile installation.
What Not to Do
- Do not replace the hybrid battery based only on C1310.
- Do not replace the hybrid battery based only on U0100.
- Do not assume U0100 was caused by the 12-volt battery.
- Do not assume the ECM is bad because communication with it was lost.
- Do not assume the absence of P0A80 proves the hybrid battery is healthy.
- Do not clear communication codes before saving the complete scan.
- Do not diagnose C1310 before looking for the primary hybrid-system fault.
- Do not continue driving if braking, propulsion, or vehicle control feels abnormal.
What to Do Next
- Save the complete scan report.
- Record which module stored U0100 and any information code.
- Test the 12-volt battery and module power supply.
- Diagnose the CAN communication fault.
- Rescan the hybrid control and brake systems.
- Find the primary code responsible for C1310.
- Evaluate the high-voltage battery only when battery-specific codes or data support it.
- Repair the confirmed fault and rescan the vehicle.
The Hybrid Geek Is Here to Help
C1310 and U0100 can create warning lights and symptoms that look like hybrid battery failure without actually confirming a battery problem.
The Hybrid Geek team can help review the codes, test the 12-volt battery, and evaluate available high-voltage battery data. If the evidence points toward a communication or control-system problem instead, knowing that before buying a battery can prevent an unnecessary repair.
Frequently Asked Questions
What does C1310 mean on a Toyota Prius?
C1310 means the brake or skid-control ECU received a hybrid-system malfunction signal. Toyota directs diagnosis toward the hybrid control system rather than treating C1310 as the root fault.
What does U0100 mean on a Prius?
U0100 means a control module lost communication with the Engine Control Module or PCM over the CAN network.
Do C1310 and U0100 mean the hybrid battery is bad?
No. Neither code directly diagnoses high-voltage battery deterioration.
If P0A80 is not present, is my Prius battery healthy?
Not necessarily. The absence of P0A80 means that particular deterioration code is not present. Other battery faults or early deterioration can use different codes or require live-data testing.
Can a weak 12-volt battery cause U0100?
Low auxiliary voltage can interfere with module startup and communication, so the 12-volt battery should be tested when symptoms support it. U0100 itself does not prove the battery is weak.
Can U0100 cause C1310?
A communication problem can disrupt hybrid-system operation and may contribute to secondary brake-system warnings, but the complete code set must be reviewed to establish the actual sequence.
Does C1310 mean there is a brake problem?
Not necessarily. C1310 is stored in the brake ECU, but Toyota describes its trouble area as the hybrid control system.
Should CAN codes be repaired before other codes?
Toyota’s Prius diagnostic guidance states that when CAN communication DTCs and other DTCs occur together, the CAN communication problem should be corrected first before troubleshooting the other codes.
Can The Hybrid Geek diagnose C1310 and U0100?
A Hybrid Geek mobile diagnostic can help review the complete code set, test the 12-volt system, and evaluate whether the high-voltage battery is involved. More advanced CAN wiring, ECM, brake, or module repair may require an appropriately equipped Toyota or hybrid repair facility.
When should I consider hybrid battery replacement?
Battery replacement should be considered when battery-specific codes and diagnostic data confirm deterioration, not simply because C1310 or U0100 appears.