OBD-II Diagnostic Trouble Code
P0512 — Starter Request Circuit
P0512 indicates a fault in the starter request circuit. The ECM monitors the key or push-button start request input and found it in an unexpected state, such as active when the engine is running or stuck on. Exact logic varies by OEM.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Starter Request Circuit
Code
P0512
Category
Powertrain
Subsystem
Ignition
Truck-relevant
Yes
What this means for truckers
P0512 (Starter Request Circuit) — the ECM saw the start signal but circuit response was wrong. On commercial trucks, this can mean a stuck starter relay or wiring fault between key/push-button start and starter solenoid.
Before you replace parts — try this first
P0512 on the Ignition system is most often faulty ignition switch or start button — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Engine may not crank, may crank intermittently, or the starter may stay engaged after start. Some trucks show a crank-inhibit message.
Common causes
- CommonFaulty ignition switch or start button ($100–$400)
- CommonWiring or connector fault in the start request circuit ($100–$350)
- OccasionalNeutral safety or clutch switch fault ($80–$300)
- OccasionalFaulty starter relay ($30–$150)
How to fix
Monitor the start request input on a scan tool while operating the key
When to use: To confirm the input changes correctly
Check the ignition switch output and wiring to the ECM
When to use: When the input does not change
Test neutral/clutch switches and starter relay
When to use: When the crank request is present but cranking is inhibited
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0500 — Vehicle Speed Sensor Malfunction
- → P0501 — Vehicle Speed Sensor Range/Performance
- → P0502 — Vehicle Speed Sensor Low Input
- → P0503 — Vehicle Speed Sensor Intermittent/Erratic/High
- → P0506 — Idle Control System RPM Lower Than Expected
Frequently Asked Questions
Can I drive with code P0512?
P0512 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0512 on a commercial truck?
Total repair cost ranges $30–$400 depending on the root cause. The cheapest fix is usually the first one listed below — Faulty ignition switch or start button — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0512 fail a DOT inspection?
P0512 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0512 and related codes?
P0512 is closely related to: P0500 (Vehicle Speed Sensor Malfunction), P0501 (Vehicle Speed Sensor Range/Performance), P0502 (Vehicle Speed Sensor Low Input), P0503 (Vehicle Speed Sensor Intermittent/Erratic/High). These codes share the same subsystem (ignition) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0512 is set without the companions, the issue is more isolated.
How do I prevent P0512 from coming back after a repair?
For P0512 (Starter Request Circuit), prevention centers on the ignition system. Use OEM-spec injectors and spark plugs (gas). Address fuel quality issues immediately — bad fuel kills injectors. Cylinder balance test annually for early warning.