P0891 — TCM Power Relay Sense Circuit High

OBD2 code P0891 flags a problem with the sensor and actuator circuits (TCM power relay sense circuit high). The car is usually drivable while you plan the repair. Common culprits: faulty sensor or actuator of the affected circuit, blown fuse feeding the circuit, corroded, loose or damaged connector.

powertrain (engine/transmission)

Is it safe to drive?

Short-term driving is OK In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm P0891 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the TCM power relay sense for damage, corrosion, chafing or poor contact.
Subsystem check With the TCM power relay sense disconnected, measure signal voltage at the harness connector: a high reading with the component unplugged indicates a short to power in the harness; if it drops to normal, the component itself is faulty.
Subsystem check Measure reference voltage, ground and signal continuity at the component connector.
Subsystem check Wiggle-test the harness while watching the signal in live data — dropouts reveal broken wires.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P0891 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for P0891 (parts and labor, USD, independent shop rates): Control module repair: $300–$1,000; Replace the faulty sensor or actuator: $150–$500; Repair wiring or connectors: $80–$250. Overall, fixing P0891 usually costs between $80 and $1,000. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

How do I diagnose P0891 myself?

Start with a scan: confirm P0891, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the TCM power relay sense, and verify the suspect component against live data before buying parts.

Will P0891 clear itself?

Sometimes P0891 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

What are the symptoms of P0891?

Drivers most often notice: increased fuel consumption in some cases, Check Engine Light on, possible limp mode depending on the affected component, the affected system operates intermittently or not at all.

What is the most common cause of P0891?

The most common causes are: faulty sensor or actuator of the affected circuit, blown fuse feeding the circuit, corroded, loose or damaged connector.

Is it safe to drive with P0891?

In most cases the car remains drivable, possibly with reduced performance or a disabled auxiliary system. If symptoms include stalling, no-start or brake/steering warnings, treat it as urgent.

When your check engine light comes on and a scan tool shows P0891, you're looking at a fault inside the transmission control module's power relay sense circuit. The PCM or TCM constantly watches this circuit while the engine is running. If it detects voltage sitting persistently above the normal operating range — usually near full battery voltage — it flags the problem and stores the code. P0891 specifically means the voltage on that sense line is reading high when it shouldn't be.

The sense circuit exists to confirm the TCM power relay is actually turning on and off as commanded. When a short, bad ground, or failed component forces the sense line to stay near battery voltage, the module thinks the relay is permanently engaged or that something is pulling the circuit high. After verifying the fault through multiple drive cycles — the exact number varies by manufacturer — the PCM locks in P0891 and illuminates the MIL. The freeze frame data captured at that moment shows engine speed, load, temperature, and vehicle speed, giving you a snapshot of exactly what was happening when the failure occurred.

Symptoms of P0891

The Check Engine light stays on after startup
Fuel economy drops noticeably in some vehicles
The transmission may shift harshly, erratically, or refuse to shift at all
Limp-home mode activates, limiting gears and performance
The system controlled by the TCM — such as shift solenoids or torque converter clutch — works intermittently or not at all

Common Causes

Most of the time, P0891 traces back to one of these issues:

Failed TCM power relay sense circuit pulling the signal high due to internal damage
Signal wire shorted to battery positive somewhere in the harness
Open ground return for the TCM power relay sense, leaving the circuit unable to reference properly
Corroded, loose, or damaged connectors at the TCM or affected component
Faulty sensor or actuator downstream in the same circuit
Poor ground connection at the module or related ground point
Open or shorted wiring within the circuit itself — chafing, melting, or broken conductors
Control module driver fault — the PCM or TCM internal output has failed

How to Diagnose P0891

Follow a logical order to avoid replacing good parts:

Connect an OBD2 scanner, confirm P0891 is active, and record all freeze frame data. This shows engine conditions when the fault triggered and helps you reproduce the failure during testing.
Check for related diagnostic trouble codes. P0800–P0805 and others in the transmission family often point to the root cause. Also search for TSBs (technical service bulletins) specific to your make and model — manufacturers sometimes publish known fixes or software updates for persistent P0xxx codes.
Perform a thorough visual inspection of the wiring harness and connectors tied to the TCM power relay sense circuit. Look for melted insulation, broken wires, corrosion buildup, and pins that are bent or pushed out of position.
Unplug the TCM power relay sense component and measure voltage at the harness side of the connector. If you still read high voltage with the device disconnected, the harness itself has a short to power. If the voltage drops to normal with the device unplugged, the component is the culprit.
Check reference voltage, ground, and signal continuity directly at the component connector using a multimeter. Compare results to manufacturer specifications if available.
Wiggle-test the harness while monitoring live data on the scanner. Any dropouts or signal interruptions reveal broken wires that only fail under motion or vibration.
Clear the code and complete a full drive cycle, including the conditions shown in the freeze frame data. If P0891 returns, the fault is confirmed and you can proceed with repairs.

Repair Options and Costs

Tackle the cheapest possible cause first:

Wiring or connector repair: $80–$250
Sensor or actuator replacement: $150–$500
Control module repair or replacement: $300–$1,000

Labor rates at independent shops typically run $75–$125 per hour. Wiring repairs and connector cleaning are usually quick jobs, while module replacement can require programming or adaptation procedures.

Total Cost to Fix P0891

Expect to pay somewhere between $80 and $1,000 depending on which part failed. The majority of cases involve damaged wiring or a bad connector — inexpensive fixes. Control module replacement is the priciest scenario and should be your last resort after confirming no other cause exists. Spending time on an accurate diagnosis before buying expensive parts almost always saves money on this particular code.

Can You Drive With P0891?

For most drivers, the vehicle stays operational, though you may notice sluggish acceleration, poor shifting, or an illuminated limp-mode indicator. Auxiliary systems related to the TCM could be disabled. However, if the fault causes stalling, a no-start condition, or affects brake or steering assist, treat it as a safety issue and stop driving immediately. Have the car towed to a shop for diagnosis.