P0507 on a Duramax LB7: Idle Speed Higher Than Expected and What Actually Causes It
TL;DR: P0507 is the generic powertrain code for idle speed running higher than the control module commanded. On a gasoline engine it points at airflow the module did not ask for. The 6.6 Duramax LB7 does not meter idle that way: it is a compression-ignition engine with no spark plugs, no coils and no idle air control valve, and its idle speed is set by the fuel quantity the module commands to the injectors. That changes the entire diagnostic order. On an LB7 the answer is almost always that the module is deliberately commanding a higher idle - because of an input it is reading, a mode it thinks it is in, or a load it is compensating for - not because air is sneaking in somewhere.
What P0507 actually means, and why the generic definition misleads on a diesel
Scan tools pull the generic description from a shared code list, so on a Duramax you will very often see P0507 rendered as an idle air control fault. Read past that wording. The underlying condition the code describes is simply this: actual engine speed at idle sat above the target idle speed by more than the allowed margin, for long enough for the module to store a fault.
That distinction matters because it tells you where not to spend your afternoon. Chasing an idle air control valve, a throttle plate or an unmetered air leak is a gasoline-engine reflex, and it will waste your time here. The LB7 has no idle air control motor, and it does not use the electronic intake air throttle valve fitted to later emissions-era Duramax engines - confirm what your specific truck is equipped with before you go looking for one, because the family changed hardware substantially across LB7 through L5P.
The other thing worth doing before any wrenching: pull the manufacturer-specific definition and the exact enable criteria for the code from GM service information for your build. Generic definitions are a starting point, not evidence.
Sensors, connectors and the small parts most of this diagnosis lands on are grouped under engine sensors, and everything LB7-specific sits under Duramax LB7 parts.
How an LB7 sets idle speed in the first place
The LB7 is a common-rail direct-injection diesel. There is no throttle plate metering the air the engine breathes at idle; the engine draws essentially the same air whether it is idling or working. Engine speed comes from how much fuel the module tells the injectors to deliver and when.
So the module runs a closed loop: it has a target idle speed, it watches actual speed off the crankshaft position sensor, and it trims commanded fuel to hold the target. Target idle speed is not one fixed number. It moves according to conditions the module is reading - coolant temperature, ambient temperature, battery voltage and electrical load, transmission range, accessory load, and whether any fast idle or power-take-off mode is active.
That is the key insight for this code. If actual idle is high and stable, the module is very likely commanding it, because the closed loop would otherwise pull fuel out until speed came down. Your job is to find out which input has convinced it to command a higher target - or, less commonly, what is adding speed the loop cannot remove.
The causes that actually turn up on an LB7
A warm-up idle that never ends. The module raises target idle when the engine is cold and lowers it as coolant temperature comes up. If the coolant temperature signal is reading colder than the engine really is - a drifting sensor, a corroded connector, a wiring fault, or a cooling system that genuinely is not reaching operating temperature - the module keeps commanding the elevated cold idle indefinitely. This is the single most common cause worth ruling out first. Compare the scan-tool coolant temperature reading against an infrared reading at the thermostat housing after a proper warm-up. A mismatch is your answer.
A fast idle or PTO mode the truck thinks is active. These trucks support elevated idle for warm-up and for driven equipment. A stuck switch, a wiring short, an aftermarket upfit harness, or a remote start module can leave the module in that mode. Look at the scan data for the commanded idle target itself: if the target has moved up, the module is doing exactly what it was asked, and you are looking for what asked it.
A pedal signal that is not truly at rest. The accelerator pedal position signal tells the module whether the driver is asking for anything. A pedal assembly that does not return fully, a floor mat or debris behind it, a worn pedal sensor, or connector corrosion can present a small but non-zero request that the module honours. Watch the pedal position values on a scan tool with your foot off the pedal, then cycle the pedal through its full travel and back and see whether it returns to the same rest value every time.
Electrical load compensation. The module raises idle to keep the alternator turning fast enough under load. A failing battery, poor grounds, high parasitic draw or an aggressive aftermarket electrical load can leave idle elevated more or less permanently. Check battery and charging system health properly before condemning anything else - on a dual-battery diesel this is a real and frequently missed cause. Monitoring hardware lives under gauges and monitoring.
Programming that is not stock. Aftermarket tuning, a module swap, or an incorrect calibration can change idle strategy directly. If the truck has a history you cannot account for, verify the calibration before you diagnose around it. Module and harness parts are grouped under harnesses and PCM/ECM.
Injector condition - but read this one carefully. The LB7 is well known for injector problems, and injector balance rates are a standard part of any LB7 workup. But failing injectors typically produce a rough, surging or unstable idle rather than a cleanly elevated one. Treat balance rates as supporting information here, not as your first suspect for a steady high idle. If the idle is high and unstable, the picture changes and injector data moves up the list. Injector and fuel system parts are under fuel injectors and fuel system.
Diagnostic order that respects how the engine works
| Step | What you are checking | What it rules out |
|---|---|---|
| 1 | Scan data: commanded idle target vs actual engine speed | Separates "module is asking for high idle" from "engine is running away from the target" |
| 2 | Coolant temperature reading vs measured temperature at the housing | Stuck warm-up idle from a cold-reading sensor or a cooling system not reaching temperature |
| 3 | Fast idle / PTO / remote start status and their switches and wiring | A commanded mode nobody knew was active |
| 4 | Accelerator pedal position at rest and through full travel | A non-zero driver request |
| 5 | Battery, grounds and charging system under load | Electrical load compensation raising idle |
| 6 | Calibration and any aftermarket modules or upfit wiring | Non-stock idle strategy |
| 7 | Injector balance rates and return volume | Fuelling faults, particularly if idle is also unstable |
Work top to bottom. Steps 1 through 4 are free and take minutes, and in shop practice they resolve the large majority of these. Typical diagnostic labour for a structured workup like this is well under an hour before you are into component-specific testing.
What not to do
Do not clear the code and road test as a diagnostic step - you lose the freeze frame data, which tells you the conditions under which the fault set and is often the fastest route to the cause. Record it first.
Do not hunt for an intake air leak the way you would on a gasoline engine. Unmetered air is not what sets idle speed here.
Do not replace a sensor because a code names it. P0507 names a condition, not a part. Nothing in the code identifies a failed component, and the trucks that end up with three new sensors and the same high idle all started with someone treating the code as a parts list.
Do not assume the code is even current. Confirm it is present and active, not a stored history fault from a previous condition, before you spend money on it.
The broader Duramax catalogue is at Duramax 6.6 parts, and general diesel hardware is under diesel engine parts.
Frequently Asked Questions
Can P0507 on an LB7 be caused by a vacuum leak? Not the way it would be on a gasoline engine. The LB7 has a vacuum pump serving the brake booster and other accessories, but engine speed at idle is set by commanded fuel, not by metered airflow past a throttle plate. A vacuum system fault will give you brake or accessory symptoms, not a raised idle. Chase the fuelling and the module's inputs instead.
Is it safe to drive with P0507 set? A high idle by itself is not usually an immediate mechanical danger, but it is a symptom of something the module is reading wrongly or a mode it should not be in, and it can affect braking feel at low speed and drivetrain engagement. Get it diagnosed rather than living with it. If the high idle came with unusual noise, smoke, temperature or a second code, stop driving it and diagnose it properly.
Why does my LB7 idle high only when it is cold? That is normal behaviour, not a fault. The module deliberately raises target idle during warm-up. It becomes a problem only if the elevated idle never comes down after the engine reaches operating temperature, which points at the coolant temperature signal or at a cooling system that is not actually getting up to temperature.
Do bad injectors cause P0507? Rarely as a direct cause of a clean high idle. LB7 injector problems usually show as rough running, surging idle, hard starting, smoke or excessive return volume. Run balance rates as part of the workup, but do not lead with injectors on a steady elevated idle - check the module's inputs first.
Will replacing the accelerator pedal sensor fix it? Only if you have confirmed the pedal signal is actually resting above zero or behaving erratically on a scan tool. Verify the data first, and check for a mechanical obstruction behind the pedal before you buy anything - that costs nothing and catches a fair share of these.
Does an LB7 have an idle air control valve like the code description says? No. That description comes from the generic code list shared across all vehicles. The LB7 has no idle air control motor and does not use the electronic intake air throttle valve fitted to later emissions-era Duramax engines. Confirm your specific truck's configuration in service information, then diagnose the fuelling side and the module's inputs.
Sources
Written by the Core Powertrain Parts tech team from diagnostic experience on 6.6 Duramax LB7 trucks, cross-checked against the generic SAE J2012 powertrain code definitions for P0507. No torque values, capacities, intervals or part numbers are quoted in this article by design: confirm every vehicle-specific figure, the manufacturer-specific code definition and the enable criteria in GM service information for your exact year and build before you act on them, and confirm current part numbers against the manufacturer catalogue.
Questions on an LB7 diagnosis or on what fits your truck: sales@coretransmissionparts.com. Orders over $70 ship free.