P2991 – Reductant Injector “D” Control Circuit Low

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit Low

Definition source: SAE J2012/J2012DA (industry standard)

DTC P2991 indicates the powertrain control module has detected a “circuit low” condition in the control circuit for Reductant Injector “D.” In practical terms, the module is seeing a lower-than-expected electrical signal on the injector’s control path during a commanded event, or during circuit monitoring, based on its internal diagnostics. The exact injector location, naming (what “D” corresponds to), wiring layout, and enabling conditions vary by vehicle, so confirm circuit identification, connector views, and test procedures using the correct service information. Because this is an electrical fault type, accurate diagnosis depends on verifying power, ground, and circuit integrity before replacing any components.

What Does P2991 Mean?

P2991 – Reductant Injector “D” Control Circuit Low means the engine/powertrain controller has determined that the control circuit for reductant injector “D” is reading low when it should not be. “Circuit low” is an electrical description: it commonly points to an unintended path to ground, loss of power feed to the actuator/driver, excessive resistance causing signal drop, or a driver/control-side issue that results in a low input being detected. The DTC structure itself is defined under SAE conventions, but the meaning here is taken strictly from the official definition: a low electrical condition in the reductant injector “D” control circuit.

Quick Reference

• Subsystem: Reductant injector “D” electrical control circuit (actuator driver circuit and associated wiring/connectors).
• Common triggers: Short-to-ground on the control wire, open power/feed to the injector/driver, high resistance in terminals, poor ground path, or a fault in the control module driver.
• Likely root-cause buckets: Wiring/connector damage, power/ground supply issue, reductant injector electrical fault, control module driver/circuit fault (varies by vehicle).
• Severity: Usually emissions-impacting; may trigger reduced performance strategies depending on platform, but typically not an immediate safety hazard by itself.
• First checks: Confirm the correct injector “D” circuit, scan for related power/voltage codes, inspect harness routing and connectors, and verify basic power/ground integrity before parts.
• Common mistakes: Replacing the injector without proving a circuit low cause, overlooking connector pin fit/corrosion, and skipping power/ground or voltage-drop checks.

Theory of Operation

A reductant injector is an electrically controlled actuator used to meter reductant into the exhaust aftertreatment system. The control module commands the injector on and off (strategy varies by vehicle) and monitors the electrical behavior of the control circuit to confirm it is operating within expected limits. This monitoring can include checking the circuit state when the injector is commanded on, when it is commanded off, or during dedicated circuit tests.

When the control circuit is pulled lower than expected, the module interprets it as a “circuit low” fault. Common electrical reasons include a short-to-ground on the control wire, a loss of the intended power feed, excessive resistance at terminals or splices causing a voltage drop, or an internal driver issue. Because designs differ, the correct interpretation depends on verifying whether the circuit is low-side or high-side controlled using service information.

Symptoms

• MIL/Check engine light: Illumination after the fault is detected and stored.
• Emissions-related warnings: Messages or indicators related to aftertreatment operation (presentation varies by vehicle).
• Reduced power strategy: Possible torque limiting or performance reduction on some platforms if aftertreatment dosing is inhibited.
• Fault recurrence: Code may reset quickly after clearing if the circuit low is hard/continuous.
• Readiness/inspection failure: Emissions monitors may not complete or may report a failure condition.
• Additional related DTCs: Other aftertreatment or electrical supply codes may appear if the underlying issue affects shared power/grounds.

Common Causes

• Wiring fault on the reductant injector “D” control circuit (short-to-ground, chafed insulation, pinched harness)
• Open power/feed to the injector (blown fuse, relay issue, open splice, poor terminal tension causing a voltage drop under load)
• Corroded, backed-out, spread, or contaminated terminals at the injector “D” connector or at the module-side connector
• High resistance in the control circuit (partially broken conductor, corrosion in a splice, water intrusion) pulling the commanded signal low
• Reductant injector “D” internal electrical fault (coil/driver load issue) that causes the circuit to read low during actuation
• Shared power or ground problem affecting multiple reductant components (common feed/ground point with excessive resistance)
• Module driver/circuit fault for injector “D” control (less common; verify wiring and load first)
• Harness routing/retention issue causing intermittent contact with ground or vibration-related low input

Diagnosis Steps

Tools typically needed include a scan tool with bi-directional controls and data logging, a digital multimeter, and a wiring diagram from service information. A test light may help with quick load checks, and back-probing pins or breakout leads reduce connector damage. If available, use a fused jumper lead and approved terminal tools for connector inspection and tension checks.

1. Confirm the DTC and capture scan tool data: record all stored and pending codes, freeze-frame, and monitor status. Note any additional reductant/aftertreatment or power supply codes that may indicate a shared feed/ground issue.
2. Verify the correct circuit/component identification: use service information to locate “reductant injector D” and its connector pinout, power supply source, ground strategy (if applicable), and the module pin controlling it (varies by vehicle).
3. Perform a visual inspection before clearing anything: inspect the injector “D” connector for damage, fluid intrusion, corrosion, terminal spread, or poor retention. Follow the harness for rubbing, pinch points, exhaust/heat damage, or contact with sharp brackets that could short the control wire to ground.
4. Clear codes and run a controlled retest: clear the DTC, then command the injector using the scan tool output control (if supported) while logging the related PID(s) for injector command and feedback/status. If the code resets immediately, prioritize circuit tests over mechanical checks.
5. Check for an unintended short-to-ground on the control circuit: with the system powered down as directed by service information, disconnect the injector connector and (if required) the module connector. Measure the control circuit for continuity to ground. If it shows a short, isolate by disconnecting intermediate connectors and inspecting harness segments until the short location is found.
6. Verify power/feed integrity to the injector: with connectors reattached as appropriate and the circuit enabled per service information, confirm the injector power supply is present when it should be. If power is missing or drops during actuation, trace upstream to the fuse/relay/splice points and correct the open or high resistance condition.
7. Perform voltage-drop testing under load: command the injector on (or use an approved load method) and perform voltage-drop checks across the power feed path and the ground/return path (as applicable). Excessive drop indicates high resistance in wiring, terminals, splices, or grounds that can pull the effective control signal low.
8. Check connector terminal fit and tension: use appropriate terminal tools to verify terminals are not spread, pushed back, or loose. Repair/replace terminals as needed. Pay close attention to any shared connectors or junctions used by multiple aftertreatment components.
9. Evaluate the injector “D” electrical load: with the injector disconnected, check for signs of internal electrical failure using manufacturer-approved procedures (do not rely on universal “normal” values). If the load is out of specification or shows intermittent open/short behavior when gently moving the pigtail, replace the injector/pigtail as applicable.
10. Perform a wiggle test with live logging: while commanding the injector and logging data, gently manipulate the harness and connectors from the injector back toward the module. If the circuit low condition appears/disappears with movement, focus on the exact area that triggers the change and repair the harness/connector issue.
11. Assess module driver only after circuit integrity is proven: if power, ground, wiring, terminals, and injector load all test good and the circuit still reports low during commanded operation, follow service information for module-side pin tests and any required confirmation steps before condemning the control module.
12. Finalize the repair with a verification drive cycle: clear codes, run the injector output test again, and complete the appropriate monitor enable conditions to ensure the DTC does not return and no related aftertreatment codes appear.

Professional tip: A “circuit low” result is often caused by a wiring/terminal issue that only shows up when the injector is commanded on and current flows. Prioritize load-based testing (output controls and voltage-drop checks) over static continuity checks, and always inspect for shared power/ground points that can make multiple components look faulty at once.

Possible Fixes & Repair Costs

Repair costs for P2991 vary widely because the underlying cause can range from a simple wiring issue to a component or control-module fault. Final cost depends on correct diagnosis, parts replaced (if any), labor time to access the injector and harness, and post-repair verification.

• Repair wiring faults: Locate and fix shorts-to-ground, rubbed-through insulation, damaged conductors, or poor splices in the Reductant Injector “D” control circuit.
• Service connectors: Clean and dry affected connectors, correct terminal push-out or poor pin fit, and restore proper locking/strain relief at the injector and module ends.
• Restore power/ground integrity: Repair shared feeds/grounds or related circuit issues that can pull the injector control circuit low under load (verify with voltage-drop testing).
• Replace reductant injector “D”: Replace only if electrical checks indicate the injector coil/actuator is internally shorted or fails manufacturer-specified tests.
• Address harness routing: Re-route and secure the harness to prevent repeat chafing/heat damage; replace protective loom or clips as needed.
• Module-side repair: If all external circuit checks pass, follow service information for control-module circuit testing; repair/replace only when confirmed by pinpoint tests.

Can I Still Drive With P2991?

You can often drive short distances with P2991, but it depends on how the vehicle manages emissions-related faults and whether additional warnings appear. Because this is a control circuit low condition for Reductant Injector “D,” the system may limit reductant dosing and may trigger reduced-power strategies on some platforms. If you notice reduced power, severe drivability changes, or any safety-related warnings (brake/steering), avoid driving and have it diagnosed promptly. If the engine runs normally, drive conservatively and schedule service soon.

What Happens If You Ignore P2991?

Ignoring P2991 can lead to continued malfunction of the reductant injection function and repeated warning lights. Over time, the vehicle may escalate to more restrictive operating modes, fail emissions inspections where applicable, and potentially set additional related DTCs as the control module detects ongoing circuit low behavior and system performance impacts.

Related Reductant Injector Codes

Compare nearby reductant injector trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P2987 – Reductant Injector “C” Control Circuit Low
• P2057 – Reductant Injector Circuit Low Bank 2 Unit 2
• P2054 – Reductant Injector Circuit Low Bank 1 Unit 2
• P2051 – Reductant Injector Circuit Low Bank 2 Unit 1
• P2048 – Reductant Injector Circuit Low Bank 1 Unit 1
• P2908 – Exhaust Aftertreatment Fuel Injector Circuit Low

Key Takeaways

• P2991 is a circuit low fault: It indicates the Reductant Injector “D” control circuit is being pulled low electrically, not a guaranteed mechanical failure.
• Wiring/connector issues are common: Shorts-to-ground, damaged insulation, and terminal problems are frequent root causes to rule out first.
• Test before replacing parts: Confirm the fault with circuit checks, load/voltage-drop testing, and connector inspections.
• Severity is usually moderate: Drivability may be normal, but emissions control may be reduced and some vehicles may limit power.
• Verify the fix: Clear the code and confirm the monitor runs without returning under similar operating conditions.

Vehicles Commonly Affected by P2991

• Diesel-equipped vehicles that use a reductant injection system for aftertreatment
• Vehicles with multiple reductant injectors or labeled injector channels (A/B/C/D) in control logic
• Applications with exposed underbody harness routing where wiring is prone to abrasion, impact, or corrosion
• Vehicles operated in wet/salty environments that accelerate connector corrosion and terminal fretting
• High-vibration duty cycles that can loosen terminals or fatigue wiring near connectors
• Vehicles with recent exhaust/aftertreatment service where connectors may be left partially seated or harnesses misrouted
• High-mileage vehicles with aged insulation, brittle loom, or previous harness repairs

FAQ

Does P2991 mean the reductant injector “D” is bad?

No. P2991 only tells you the control circuit for Reductant Injector “D” is detected as low electrically. A short-to-ground, poor connector contact, or a power/ground issue can produce the same result. Replace the injector only after circuit testing confirms it is the cause.

What electrical problems most commonly cause a “circuit low” code like P2991?

Common causes include a short-to-ground in the control wire, damaged insulation contacting metal, water intrusion or corrosion in a connector that pulls the signal low, or a power/feed problem that prevents the actuator from being driven correctly and results in a low reading during monitoring. Exact logic varies by vehicle, so confirm with service information.

Can low fluid level cause P2991?

Low fluid level is not a direct electrical “circuit low” condition. While low fluid can cause other system faults, P2991 specifically points to the Reductant Injector “D” control circuit being low. Treat it as an electrical diagnosis first unless service information for your vehicle indicates otherwise.

Will clearing the code fix P2991?

Clearing the code only removes the stored fault information temporarily. If the circuit low condition is still present, the code will typically return when the control module reruns the monitor. Clear codes only after inspections/repairs, then verify by running the conditions required for the monitor to complete.

What should be checked first for P2991?

Start with a visual inspection of the injector “D” connector and harness routing for damage, corrosion, loose terminals, or chafing. Then confirm the fault with scan data and perform circuit testing (including voltage-drop under load) to identify where the circuit is being pulled low before replacing any components.

For a lasting repair, treat P2991 as an electrical circuit-low diagnosis: verify the fault, isolate shorts-to-ground or connection problems, confirm power/ground integrity under load, and only replace the injector or module after tests prove they are responsible.