2006 Nissan Frontier P0603 Fault: Engine Stalls, Poor Idle, Black Smoke - Diagnosis and Fix

P0603 indicates an ECM (Engine Control Module) power supply issue. First, verify the relevant fuses—especially fuse #53 (20A) in the engine fuse box. Check for proper voltage on both terminals using a digital multimeter. Ensure you have access to wiring diagrams and a DVOM (digital voltmeter). Do you own an OBD11 scanner or a basic code reader? Also, confirm if your truck has any aftermarket modifications that might interfere with the system.

All fuses have been checked and are functioning properly. I am familiar with reading wiring diagrams and currently use my own multimeter. I only have a basic OBD11 code reader, and the truck has no modifications—no added electronics or performance chips. The only fault code detected was P0603, related to fuel system operation. Thanks for your help, Gabe.

You mentioned both P0603 and a fuel-related issue—was there any other diagnostic trouble code (DTC) present besides P0603?

Only P0603 was recorded on the OBD11. Both codes were cleared from the scanner, and no further issues were detected at that time.

Currently, there are no active codes. However, performance remains poor—idle is unstable and black smoke is visible during operation. Did you notice any other fault codes? If only P0603 was present, begin with a fuel pressure check. Perform a KOEO (Key On, Engine Off) test to observe the fuel pressure drop over five minutes.

Correct—there are no active codes now, and performance remains poor. I did not mention earlier that when I unplug the throttle body sensor while the engine is running, there's no change in behavior. I will proceed with a fuel pressure test immediately.

Unplugging the throttle position sensor should trigger a new fault code—verify this using your scanner to confirm. If no code appears after disconnection, it may point to an ECM power issue or communication failure. The absence of a response suggests potential PCM malfunction.

After unplugging the throttle body sensor, no new fault code was detected on the scanner. This indicates that the sensor itself is likely not the root cause—suggesting instead an issue with the ECM's power supply or internal function.

When I fully unplug the ECM connector, the engine fails to start at all. This strongly suggests a potential failure in the ECM module itself, rather than a throttle sensor malfunction.

I tested the throttle position sensor (TPS) while the key was on and the engine off: Pin 1 reads 4.8V, Pin 2 reads 5.18V, and ground pin shows 37.4 ohms. A critical observation is that during idle, the engine runs with heavy black smoke from the exhaust—this points to possible fuel delivery or combustion problems.

Which engine variant? 4-cylinder QR or 6-cylinder VQ? Is this a vehicle equipped with electric throttle control (ETC)? Are there any other codes such as PO12x present? Perform a fuel pressure test and conduct a leak-down test. Also, check the engine oil—excess fuel consumption may indicate leaking injectors that are dripping into the oil pan. Black smoke and oil contamination are strong signs of internal issues. Confirm whether P0603 is still active or if it has been cleared.

It's a 6-cylinder VQ engine with electric throttle control (ETC). Only P0603 was present—no other codes. I currently need to acquire a fuel pressure gauge for testing.

Electric throttle systems use redundant sensors (typically two TPS)—if no code is triggered, it's likely safe to leave the system as-is. Persistent black smoke and high fuel consumption are red flags. Perform a fuel pressure test and conduct a leak-down test. Check engine oil for signs of fuel contamination—this often occurs when injectors fail and allow excess fuel into the oil pan. Also confirm whether P0603 is still active or has been cleared.

P0603 troubleshooting step 1: Check fuse #53 (20A) in the engine fuse box (IPDM). Verify voltage on both terminals—ensure it's at least 12V. This fuse powers the ECM and is critical for proper operation.

I'm unsure which of the three 20A fuses corresponds to #53. With the key on and engine off: - EGI (20A): 12.5V on both sides - ETC (20A): 12.5V on both sides - FR FOG LAMP (20A): no voltage—this is unrelated.

P0603 is a top-priority issue. Fuse #53 appears to be functional, as it shows 12V on both terminals. The next step is to verify if the ECM terminal 121 receives proper 12V power—this test will help determine whether the PCM is receiving adequate voltage or if the module itself is faulty.

Confirmed: Fuse #53 has consistent 12V on both terminals. I will now check terminal 121 of the ECM for a stable 12V supply.

The issue was resolved by swapping the ECM relay with the front fog lamp relay. After the swap, the truck runs normally—idle is stable, no black smoke, and all systems are functioning properly. This suggests a faulty relay in the ECM circuit was causing intermittent power loss to the PCM.