Street Smart - May - 2016

Circuit Diagnosis Made Easier by Looking at the Big Picture

Cadillacs have always given diagnosticians headaches when it comes to diagnostics. Let’s not kid anyone here: the Cadillac has twenty plus modules, miles of wiring, and controls for just about everything including the kitchen sink.

So when it comes to diagnostics, the trick is to keep it simple. Here we’re going to talk about a 2005 Cadillac STS with the MIL lit; the only complaint was that it wouldn’t pass the emissions (smog) test.

First things first: If you can’t verify the complaint, perform a quick scan test and check for codes. In this case there were four codes: DTC P0451, P0463, P0533, and P2068.

DTC P0451 — Evaporative Control System Pressure Sensor; Range/ Performance — The control module monitors the fuel tank pressure (FTP) sensor signal for vacuum decay or excess vacuum during the evaporative emission (EVAP) diagnostic test. The control module supplies a 5-volt reference and ground to the fuel tank pressure sensor.

Conditions for setting DTC P0451:

  • Fuel tank pressure sensor signal voltage high, about 4.5 volts or more negative pressure
  • Fuel tank pressure sensor signal voltage low, about 1.5 volts or less positive pressure

DTC P0463 — Fuel Level Sensor; High Circuit Input — The primary and secondary fuel level senders change resistance in response to fuel level. The engine control module (ECM) monitors the signals from the primary and secondary fuel level senders to determine the fuel level.

When the fuel tanks are full, the resistances of both fuel level senders are high and the ECM senses a high voltage at the primary and secondary fuel level senders. When the fuel tanks are low, the resistances of the fuel level senders are low, and the ECM senses a low voltage.

The ECM uses these signals to calculate the amount of fuel left in the tanks. The ECM sends the fuel level signals via the GMLAN serial data circuit to the instrument cluster to control the fuel gauge. The fuel level information is also used for misfire and evaporative emission (EVAP) diagnostics. This code indicates a higher-than-normal primary fuel level sender signal.

Conditions for setting DTC P0463: Primary fuel level signal is above 4.5 volts.

DTC P0533 — A/C Refrigerant Pressure Sensor Circuit; High Input — The engine control module (ECM) monitors the high side refrigerant pressure via the A/C refrigerant pressure sensor. The ECM applies five volts and ground to the sensor. Changes in the A/C refrigerant pressure cause the signal to vary. When the pressure’s low, signal voltage is low. When the pressure’s high, signal voltage is high, and the ECM commands the cooling fans on.

When pressure is too high or too low, the ECM won’t allow the A/C compressor clutch to engage and it sets code P0533. With the DTC set, the ECM won’t allow the A/C compressor clutch to engage. This is to protect the compressor.

Conditions for setting DTC P0533: A/C refrigerant pressure sensor signal voltage is above 4.88 volts for more than three seconds.

DTC P2068 — Fuel Level (Sensor B) Circuit High — The primary and secondary fuel level senders change resistance in response to fuel level.