Close Is Not Good Enough

Posted 6/10/2010
By Paul Kujawa

Vehicle: The vehicle referred to in this article is a 2004 Nissan Titan; however, this problem could occur on just about any vehicle.

Symptom: This vehicle came in as a crank, no-start, and a stored sensor power supply code P1229. Code P1229 refers to the 5-volt reference used by the accelerator pedal position (APP) sensor 1, APP sensor 2, the evaporative emissions (EVAP) control system pressure sensor, the A/C refrigerant pressure sensor and the power steering pressure sensor. The 5-volt reference for the APP sensor checked good but after the code P1229 was cleared, it would come right back.

Diagnosis: Failure of the engine control module (ECM) relay is very common and causes low voltage supply to the ECM, so a test of the battery voltage supply to the mass air flow (MAF) sensor's brown wire was performed and it showed good battery voltage. This is a convenient place to test power from the ECM relay. The ECM relay was also swapped with a matching relay in the intelligent power distribution module (IPDM), but that yielded no answers either.

Testing for the no-start problem resumed. There was no security light, so it was not an immobilizer system problem. Attempting to check for engine rpm while cranking, the scan tool would stop communicating, so that was no help. The fuel pressure was 51 psi, which is good. Fuel quality was also tested. The engine had good spark and injector pulse. The spark plugs were getting wet, and we found that with the fuel pump fuse removed, the engine would start and run if supplemental fuel was supplied. The fuel pump fuse was reinstalled while it was running on supplemental fuel and then the engine stayed running. At this point it would not accelerate above 1200 rpm due to the P1229 power supply code for the APP sensor. The engine would lose spark at times as well.

With the P1229 code coming back instantly, the focus was again back on this code. The next step was to check all of the 5-volt reference circuits at the ECM terminals 48, 49, 68, 90 and 91. If any 5-volt reference circuit was low, the sensor(s) receiving that reference was to be unplugged to see if it would return. A low circuit would indicate a possible defective sensor shorting out the 5-volt reference.

5-volt reference wires checked:

• EVAP control system pressure sensor: Sky Blue wire was checked at terminal 48 of the ECM.

• A/C refrigerant pressure sensor: Red/Yellow wire was checked at terminal 49.

• Power steering pressure sensor: White/Blue wire was checked at terminal 68.

• APP1 sensor: Blue wire was checked at terminal 90.

• APP2 sensor: White/Red wire was checked at terminal 91.

All of the 5-volt references were checked and confirmed to be OK, as were the ECM powers and grounds. The ECM was then replaced but yielded no fix. The same starting problems were present, and the same P1229 code reset instantly with the key "on." When the 5-volt references were rechecked at the ECM, all the circuits measured 3.8 volts with the exception of terminal 91, which read 5.0 volts. The technician working on the vehicle thought that 3.8 volts was "close enough" to 5.0 volts, so therefore he told us that the 5-volt references were good. Wrong! The related sensors were then unplugged one at a time as the voltage was being monitored. When the A/C refrigerant pressure sensor was unplugged, the full 5-volt reference returned and the engine started and ran fine. The A/C pressure sensor was replaced and the vehicle was fixed.

We have seen a shorted 5-volt reference several times on many different vehicles, commonly resulting in a no-start with no spark, no injector pulse, no malfunction indicator light on, and no communication with the scan tool - basically shutting the ECM down. In this case, the 5-volt reference was not pulled all the way to ground so it did not cause the ECM to completely shut down, but it clearly affected its operation.

If you encounter this problem, remember to watch out for a partially shorted 5-volt reference and don't assume that 3.8 volts is close enough.

Paul Kujawa is an Identifix Asian specialist. He is a Nissan master and ASE master with L1 certification.

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