OBD-II trouble code
P2059: Reductant Injection Air Pump Control Circuit/Open
Only one kind of SCR system can set this code at all: the air-assisted sort, which blows compressed air through the injector to atomise the fluid. Airless systems have no pump to command, and knowing which one you own settles most of the diagnosis before it starts.
Quick facts
- System
- Powertrain
- Category
- Exhaust / Aftertreatment
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $130 – $1,500
- DIY difficulty
- Advanced DIY
Browse every code in P2000–P20E8, or start from the full code library.
What does P2059 mean?
There are two ways to get diesel exhaust fluid into an exhaust stream as a fine mist, and they lead to different hardware. The common approach today is hydraulic: a supply pump raises the fluid to a few bar and a precise nozzle does the atomising on its own. The other approach, used on a number of earlier and mid-generation light-duty diesels and on several commercial applications, feeds the injector a stream of compressed air and lets the air do the work of breaking the fluid into droplets. That second design needs a dedicated electric air pump, and P2059 is the code for the module losing electrical control of it.
So the first and most valuable thing this code tells you is architectural. If the vehicle uses air-assisted dosing, there is a small electric compressor somewhere on the frame or in the engine bay with an air line running to the injector, and the code is about the command path to it. If the vehicle does not use air assist, there is nothing to command, no pump to find, and the code has almost certainly arrived via a generic scan tool putting a manufacturer-specific fault onto the nearest standard address. That second situation is far more common than people expect, and it is worth ruling out by finding the pump before any parts are ordered.
The word that does the most work in the title is **control**. This is a code about the circuit that switches the pump, not about the air the pump produces. The module is watching its own output — usually a low-side driver feeding a relay, sometimes a direct driver feeding the motor, occasionally a serial command to a separate dosing control unit — and reporting that the path is open, with no current flowing where current was asked for. An air system that is electrically perfect but mechanically blocked, leaking, or producing too little pressure does not set this code. It sets a pressure or flow code instead, and the two groups get confused constantly. If the pump is audibly running, this is probably not the fault to chase.
Why it matters mechanically, rather than just as a lamp on the dash, comes down to what atomisation is for. Urea solution needs heat and fine division to break down cleanly into the ammonia the catalyst actually uses. Sprayed as a mist into hot gas it hydrolyses; dribbled in as coarse droplets it lands on metal, gives up its water and leaves solid deposits behind. On an air-assisted system the air is a required ingredient in making that mist, not a refinement. An air pump that has stopped being commanded therefore does not simply reduce dosing accuracy — it invites crystalline buildup at the injector tip and in the mixing section, which is a cleaning job on top of the electrical repair. That is the reason for treating the code as urgent even though the vehicle drives normally, and it is a different reason from the one that applies to most dosing faults.
One more characteristic of this particular component is worth knowing before condemning it. The pump is a small motor running a duty cycle, which gives it the wear pattern of a motor rather than of a sensor: brushes wear, bearings dry out, and many of these pumps carry a thermal cutout that opens when the unit overheats and closes again when it cools. An intermittent open that appears after long highway runs and disappears overnight fits that behaviour, and it is not a wiring fault — it is a pump telling you it is working too hard, often because the air path downstream has partly blocked with the very deposits the air was supposed to prevent.
Common causes
- Air pump relay failed open, or its socket terminals corroded
- Open in the control wire between the module and the relay or pump
- Pump motor worn out electrically, with open brushes or a failed winding
- Pump thermal protection opening repeatedly because the air path downstream is restricted
- Corroded or water-filled connector at the pump, which is usually mounted low and exposed
- Fuse or power feed for the air pump circuit open
- Module driver output for the pump failed open
- Ground for the pump circuit corroded at its stud or body attachment
- Harness chafed or broken where it crosses the frame to the pump mounting location
- Previous repair splice in the pump circuit corroded through
Symptoms
- Warning lamp on with normal engine performance
- SCR, DEF or exhaust-system message on the dashboard
- No sound from the air pump at key-on or during a dosing event
- Inducement warnings and staged power reduction as the countdown advances
- Air pressure or dosing flow codes stored alongside
- White crystalline deposits visible at the injector tip or the mixing section when inspected
- Fault that appears after long or loaded highway driving and is gone the next morning
- DEF consumption lower than expected, or unexplained loss with no dosing credit
- SCR efficiency or NOx code following weeks later
- Pump audibly running continuously or at odd times, which usually points somewhere other than this code
Diagnostic steps
- 1.Establish first whether this vehicle has air-assisted dosing at all. Find the pump and its air line to the injector. If there is no pump, this code has most likely been mis-mapped by a generic tool from a manufacturer-specific fault and the real code should be read with a capable scan tool.
- 2.Listen for the pump with the key on and during a commanded dosing event. A pump that runs rules out most of what this code describes and redirects the diagnosis toward air pressure and flow faults instead.
- 3.Command the pump with a bidirectional tool and watch for current flow at the relay or the pump connector. No current with a command present confirms the open.
- 4.Check the fuse and relay before anything else, and swap in a known-good relay of the same part number rather than testing the old one in place.
- 5.Back-probe the pump connector for supply and ground with the pump commanded, and perform a voltage-drop test on the ground path under load. Continuity readings on a motor ground are misleading.
- 6.Measure the motor winding resistance at the pump terminals. An open winding or worn-out brush gear reads as an infinite or wildly high value and settles the question on the spot.
- 7.If the fault is intermittent and heat-related, let the vehicle reach operating temperature on a road test and retest hot. A pump whose thermal cutout is opening behaves exactly this way and is usually telling you the air path is restricted.
- 8.Inspect the air line and the injector tip for crystalline deposits. Restriction there raises the load on the pump and is both a cause of repeated thermal trips and an outcome of running without proper atomisation.
- 9.Trace the control wire from the module end with the harness flexed where it crosses the frame. This pump is mounted low on most installations and its feed takes road spray.
- 10.After repair, clear the codes, command the pump, verify it runs and that air reaches the injector, then complete a drive cycle long enough for the dosing monitor to run rather than relying on a cleared lamp.
Repair cost
$130 – $1,500
Diagnosis is $120 to $250. A relay or fuse is $20 to $90 and is the cheapest realistic outcome, which is why it should be eliminated first. Wiring and connector repairs in the pump circuit run $120 to $450, higher if the damaged section crosses the frame and needs the vehicle on a lift. A replacement reductant injection air pump is $150 to $800 in parts with 0.7 to 3 hours of labour depending on how buried the mounting is; commercial applications sit at the top of that range. Budget an extra $150 to $600 if deposits have built up and the air line, injector tip or mixing section needs cleaning, because an air-assisted system that has been dosing without air usually has some. Module driver failure is uncommon here but adds $600 upward with programming.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with reductant (def) injection air pump replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.