P0C28 — AUXILIARY Transmission Fluid Pump Motor Current High #

Note: The wording of this code may vary depending on the OEM (Original Equipment Manufacturer).

• AUXILIARY Transmission Fluid Pump Motor Current High (generic SAE / Toyota / Lexus / Honda / Acura)
• Auxiliary Transmission Oil Pump Motor Circuit High (Nissan / Infiniti)
• Electric Transmission Pump Current Too High (Hyundai / Kia)

Summary of DTC P0C28 #

DTC P0C28 indicates that the onboard computer has detected an abnormally high electrical current in the auxiliary transmission fluid pump motor circuit. This auxiliary pump is an electrically driven component that helps circulate transmission fluid, often used in hybrid or stop-start systems to maintain pressure when the engine is off. A high current draw usually points to excessive resistance, internal shorting, or a mechanical binding within the pump. Ignoring this code can lead to reduced transmission performance, unexpected drivability issues, or even internal transmission damage if not addressed. While the car may still move, the risk of further damage increases over time.

Common Symptoms for Code P0C28 #

When the P0C28 code is active, you might notice several warning signs. These symptoms can vary in severity but often signal trouble with the auxiliary transmission pump or its circuit. Watch for these issues in daily driving:

• Illuminated Check Engine Light or Hybrid System Warning
• Transmission may enter fail-safe/limp mode
• Noticeable hard or delayed shifting
• Unusual noises from the transmission area (whining, humming)
• Reduced performance in stop-start or hybrid modes
• Erratic transmission operation, especially at low speeds or when engine is off

Probable Causes for P0C28 (Most to Least Common) #

Several factors can cause the P0C28 code to set. The most frequent causes relate to the pump’s electrical circuit or the pump itself. Diagnosing the root cause quickly is important to prevent further drivetrain issues. Here’s a ranked list of possible culprits:

1. Faulty auxiliary transmission fluid pump motor (internal short or excessive wear)
2. Shorted or damaged wiring in the pump motor circuit
3. Poor electrical connections at the pump or control module
4. Defective relay or fuse (if equipped)
5. Malfunctioning transmission control module (TCM)
6. Contaminated or degraded transmission fluid causing pump overload

Step-by-Step Diagnostic Guide for P0C28 #

Diagnosing P0C28 requires both electrical and mechanical checks. Always start with safety: disconnect the battery before working on high-voltage hybrid components. Here’s a practical step-by-step approach:

• Scan for additional DTCs and record freeze-frame data to understand context.
• Inspect all wiring and connectors leading to the auxiliary pump for visible damage, corrosion, or loose pins.
• Check the pump’s fuse and relay (if applicable); replace if faulty.
• Test the auxiliary pump motor resistance using a digital multimeter; compare with OEM specs.
• Command the pump ON (with a scan tool) and monitor current draw; excessive amperage confirms internal pump issues.
• If wiring and pump are OK, check TCM outputs and related control circuits.
• Inspect fluid level and condition; low or dirty fluid can strain the pump.

Possible Repairs for P0C28 #

Once the cause is identified, repairs can range from simple to more involved. Always use OEM-approved parts and follow safety guidelines when working near high-voltage hybrid components. Typical fixes include:

• Replace the auxiliary transmission fluid pump motor if it fails resistance or current draw tests.
• Repair or replace damaged wiring or connectors in the pump circuit.
• Replace faulty relay or fuse associated with the pump circuit.
• If caused by contaminated fluid, perform a transmission fluid exchange with the correct OEM fluid.
• Reprogram or replace the TCM if a control module fault is confirmed.

Is It Safe to Drive with Code P0C28? #

Limited. While it’s often possible to drive with the P0C28 code present, it is not recommended for extended periods. The auxiliary pump is crucial for maintaining transmission pressure during stop-start or hybrid operation. If the pump fails, you risk transmission overheating, harsh shifting, or internal damage—especially in traffic or at low speeds. Schedule a repair as soon as possible, and avoid situations where the engine frequently shuts off or restarts.

Vehicles Most Commonly Affected by P0C28 #

The P0C28 code is most often seen on modern hybrids and vehicles with electrically assisted automatic transmissions. These models use an auxiliary pump to ensure fluid flow even when the main engine is off. The following vehicles report this code most frequently:

• Toyota Prius (all hybrid generations)
• Lexus ES/RX/GS hybrid models
• Honda Accord Hybrid and Acura hybrids
• Nissan Altima Hybrid, Infiniti hybrid models
• Hyundai Sonata Hybrid, Kia Optima Hybrid
• Other OEMs with stop-start automatic transmissions

Common Mistakes & Best Practices with P0C28 #

Some missteps can delay accurate diagnosis or cause further issues. To avoid headaches, remember these tips:

• Don’t ignore wiring checks: Mechanical pump failures are common, but wiring faults are also frequent and cheaper to fix.
• Don’t replace the pump without testing: Always confirm with current draw and resistance checks before condemning the pump.
• Consult OEM service info: Each manufacturer may have unique test procedures or wiring layouts.
• Clear codes after repairs: Always retest the system after completing repairs and clearing codes.
• Use proper safety gear: Hybrids carry high voltages—never shortcut safety steps.

Quick FAQ for P0C28 #

Here are answers to the most common questions about this trouble code:

• Q: Can a weak battery cause P0C28?
  A: In rare cases, a weak 12V or hybrid battery may impact circuit operation, but most often the issue is with the pump or wiring.
• Q: How much does it cost to fix P0C28?
  A: Costs vary widely: a wiring repair may be under $200, while pump replacement or TCM work can exceed $1000, especially at a dealership.
• Q: Will changing transmission fluid clear the code?
  A: Only if the pump is overloading due to fluid contamination. Most cases require electrical or pump repairs.