P2798 — Auxiliary Transmission Fluid Pump Control Circuit Low #

The wording of this code may vary depending on the vehicle manufacturer.
Known label variations:

• P2798 — Auxiliary Transmission Fluid Pump Control Circuit Low (Generic/Multiple OEMs)
• P2798 — Auxiliary ATF Pump Control Circuit Low (Toyota/Lexus)
• P2798 — Auxiliary Transmission Fluid Pump Control Signal Low (Honda/Acura)
• P2798 — Low Voltage in Auxiliary Transmission Fluid Pump Circuit (Nissan/Infiniti)

Summary of code P2798 #

P2798 indicates a problem detected by the vehicle’s Powertrain Control Module (PCM) or Transmission Control Module (TCM) in the control circuit for the auxiliary transmission fluid pump. The auxiliary pump helps maintain proper transmission fluid pressure during certain operating conditions, such as engine start/stop events or low-speed driving. This code specifically means the control circuit voltage is lower than expected, which could signal an electrical fault, wiring issue, or pump failure. While not always immediately critical, this fault can affect transmission performance and should be addressed promptly. Ignoring it may lead to more severe transmission issues over time.

Common symptoms for code P2798 #

When code P2798 is present, drivers or technicians may notice one or more of the following symptoms. These issues often point toward irregularities in the way the transmission operates, especially during low-speed or start/stop scenarios:

• Transmission warning light or check engine light illuminated
• Delayed or harsh shifting, especially at low speeds or from a stop
• Unusual transmission noises (whining, buzzing, or humming)
• Reduced fuel efficiency
• Transmission overheating in severe cases
• Erratic idle or engine stalling at stop/start operations

Probable causes for code P2798 (by frequency) #

Several factors can trigger code P2798, often related to the electrical or mechanical integrity of the auxiliary pump circuit. Here are the most common causes, listed in descending order of likelihood:

1. Faulty auxiliary transmission fluid pump (internal electrical failure)
2. Damaged or corroded wiring/connectors in the pump control circuit
3. Failed pump control relay or fuse
4. Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM)
5. Poor ground connection at the pump or module

Step-by-step diagnosis for code P2798 #

A methodical approach ensures you pinpoint the root cause of P2798 without unnecessary parts replacement. Here’s a recommended diagnostic path for technicians and advanced DIYers:

• 1. Verify the code: Use a scan tool to confirm P2798 is active and check for related codes.
• 2. Inspect wiring and connectors: Look for corrosion, damage, or loose connections at the auxiliary pump and related harness.
• 3. Check fuses and relays: Test the auxiliary pump fuse and relay for continuity and proper operation.
• 4. Test auxiliary pump operation: With proper safety procedures, use a multimeter to check for voltage at the pump connector when commanded on by the scan tool.
• 5. Check ground circuit: Ensure the pump and control module have a clean, solid ground.
• 6. Assess the control module: If all wiring and components test good, consider TCM/PCM diagnostics or reprogramming per OEM procedure.

Possible repairs for code P2798 #

Repairs should directly address the identified cause. Always use OEM parts or those meeting OEM specifications for best results. Common fixes include:

• Replace a faulty auxiliary transmission fluid pump
• Repair or replace damaged wiring/connectors in the pump circuit
• Replace a failed relay or fuse controlling the pump
• Correct poor ground connections at the pump or control module
• Reprogram or replace the TCM/PCM if diagnosed as faulty (rare)

Is it safe to drive with code P2798? #

Driving with code P2798 is not recommended for an extended period. While you may not notice immediate issues, the auxiliary transmission fluid pump is critical for maintaining fluid pressure during specific conditions. Ignoring this fault can lead to transmission overheating, harsh shifts, or even transmission failure, especially in urban driving with frequent stops. If you must drive, do so only to a repair facility and avoid heavy traffic or stop-and-go conditions.

Vehicles most commonly affected by code P2798 #

This code is most frequently seen on modern vehicles equipped with electronically controlled automatic transmissions and start/stop systems. Brands that use auxiliary transmission fluid pumps and may display P2798 include:

• Toyota/Lexus (especially hybrids and vehicles with start/stop technology)
• Honda/Acura (select models with advanced transmission control)
• Nissan/Infiniti (CVT and some traditional automatics)
• Ford (select models with dual-clutch or eco transmission systems)
• General Motors (some Cadillac and Chevrolet models)

Common mistakes & best practices for P2798 #

Some errors can prolong the repair process or cause unnecessary expense. To ensure a successful fix, keep these points in mind:

• Replacing the auxiliary pump without checking wiring — Always inspect wiring/connectors first.
• Overlooking fuses and relays — These are inexpensive and common failure points.
• Ignoring ground connections — Poor grounds can mimic component failures.
• Skipping a scan for related codes — Other codes can provide valuable clues.
• Using non-OEM parts — Always use OEM or OEM-equivalent for transmission components.

Express FAQ for code P2798 #

Even experienced technicians and owners have questions about P2798. Here are quick answers to the most common:

• Q: Can a weak battery cause P2798?
  A: Rarely. While low voltage can affect circuits, P2798 usually points to a specific issue in the pump’s circuit or the pump itself.
• Q: Is P2798 likely to return if I just clear the code?
  A: Yes. Clearing the code without repairs will not resolve the underlying problem; the code will likely reappear quickly.