P2691 Code Symptoms, Causes, Diagnosis & Repair Cost Guide Pricing

Introduction

When the power‑train control module (PCM) detects injector signals that do not match the expected parameters for cylinder 6, it stores code P2691 and illuminates the Check Engine Light. Most drivers first notice a rough idle that feels like a single‑cylinder stumble, a momentary loss of acceleration, or a sudden dip in power that can trigger a stutter while cruising. In some cases the engine may briefly stall before the PCM shuts down fuel delivery to protect the cylinder. Because the fault is tied to data communication rather than a mechanical failure, early diagnosis prevents unnecessary component replacement and avoids potential damage to the engine’s combustion balance.

Symptoms

• Rough, uneven idle that feels like one cylinder is “missing.”
• Brief hesitation or power dip when the throttle is opened, especially at low RPM.
• Intermittent loss of power that may cause the vehicle to lurch forward or coast.
• Check Engine Light (CEL) flashing or steady, often accompanied by a stored P2691 code after a scan.
• Engine may stall for a second before the PCM restores fuel, then runs normally until the next data mismatch.

These signs appear because the PCM is rejecting injector pulse‑width or timing data that it deems out of spec for cylinder 6. The module then commands a protective fuel‑cut or retards ignition to keep the engine from running under unsafe conditions.

Why This Happens

Faulty PCM Data Processing

The PCM continuously compares real‑time injector pulse‑width data against calibration tables stored in its memory. Corruption of the PCM’s firmware, a failed internal processor, or degraded memory cells can cause the module to misinterpret legitimate injector signals as “incompatible.” When this occurs, the PCM logs P2691.

Wiring or Connector Issues Between PCM and Injector Driver

A high‑resistance connection, corrosion, or a short in the harness that carries injector driver signals can distort the voltage waveform reaching the PCM. The distorted waveform falls outside the expected range, prompting the module to flag the data as incompatible.

Injector Driver Circuit Failure Within the PCM

Most modern PCMs contain integrated driver circuits that generate the high‑voltage pulses for each injector. If the driver for cylinder 6 develops an internal fault—such as a leaky transistor—the resulting pulse shape will differ from the calibrated profile, triggering P2691.

Incorrect PCM Calibration or Software Version

After a major engine‑control software update, the PCM may retain old calibration data for injector timing. If the updated software expects a different pulse‑width range, the existing data for cylinder 6 becomes “incompatible” until the module is re‑programmed with the correct calibration set.

Diagnostic and Repair Procedures

1. Retrieve the Code and Freeze‑Frame Data

– Connect a professional OBD‑II scanner. Record the exact engine speed, load, and fuel‑trim values at the moment P2691 was set. Freeze‑frame data helps confirm whether the fault occurs under specific operating conditions.

1. Inspect PCM‑Injector Wiring

– Visually examine the harness for corrosion, broken wires, or loose connectors at the PCM and injector driver. Use a multimeter to measure resistance; values should match the service manual (typically < 0.5 Ω). If resistance is high, clean or replace the connector—still treat the PCM as the primary suspect.

1. Perform a PCM Communication Test

– Run a bidirectional communication test from the scan tool. Verify that the PCM can both send and receive data on the injector driver bus (often a CAN‑high line). Communication failures indicate a PCM internal fault rather than external wiring.

1. Execute an Injector Pulse‑Width Capture

– Some advanced scan tools can display real‑time injector pulse‑width for each cylinder. Compare cylinder 6’s waveform to cylinders 1‑5. A markedly different shape or duration points to a driver circuit problem inside the PCM.

1. Re‑program the PCM

– If the hardware checks out, update the PCM’s software to the latest manufacturer calibration. Re‑flashing typically costs $150‑$250 and resolves mismatched data tables. After re‑programming, clear the code and perform a drive cycle to confirm the fault does not return.

1. PCM Repair or Replacement

– When communication tests fail, pulse‑width capture shows abnormal waveforms, or re‑programming does not clear P2691, the PCM is likely damaged. A qualified shop can attempt a board‑level repair, but reliability is uncertain. Replacement with a VIN‑matched PCM ensures correct software, immobilizer integration, and warranty coverage. Expect $800‑$1,200 for the unit plus $200‑$300 labor.

When Replacement Makes More Sense Than Repair

Modern control modules are deeply integrated with the vehicle’s security, immobilizer, and emission‑control systems. A damaged processor or compromised driver circuit often cannot be fully restored with a simple repair, and intermittent failures may reappear after a short interval. Choosing a new, VIN‑matched PCM eliminates the risk of recurring data‑incompatibility errors and guarantees that the module’s firmware aligns with the vehicle’s calibrated specifications.

Flagship One specializes in VIN‑matched control modules, providing a plug‑and‑drive solution backed by a comprehensive warranty. Because the replacement unit is pre‑programmed to your vehicle’s exact configuration, installation is straightforward for any qualified technician, and the risk of post‑repair drivability issues is minimized.

Preventive Maintenance

• Keep PCM connectors clean and dry. Periodic inspection of the PCM harness for moisture, corrosion, or loose pins prevents signal distortion.
• Use a quality OBD‑II scanner for regular code reads. Early detection of minor data anomalies can be addressed before they trigger P2691.
• Follow manufacturer‑recommended software updates. When a dealer releases a PCM calibration patch, have it applied promptly to keep injector timing tables current.
• Avoid aftermarket engine‑tuning devices that modify injector pulse‑width without PCM re‑calibration. Unapproved changes can create data mismatches that the PCM will flag as incompatible.