Diagnostic Aids

Check for any of the following conditions:

• Check for a restriction in the inlet air passage. A duct or inlet hose that collapses when hot or that is blocked by debris can cause a DTC P0172 to set.
• Check for correct fuel pressure. Fuel system will be rich if fuel pressure is too high. To determine the cause of a DTC P0172, monitoring fuel pressure while driving vehicle at various road speeds may be necessary. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
• A MAF sensor output that is incorrect may cause PCM to command fuel system to go rich. Check performance of MAF sensor. Check for air restrictions and for air inlet leaks near MAF sensor.
• Check for a damaged wiring harness. If harness appears to be OK, observe HO2S 1 display on scan tool while moving connectors and wiring harnesses related to the sensor. A change in display will indicate location of fault.
• Check for a contaminated oxygen sensor. Silicone and other contaminants can cause oxygen sensor to provide false exhaust oxygen content readings. If contamination is found, determine the cause and correct the condition before replacing HO2S.
• Check HO2S 1 for water intrusion into the wiring harness and sensor housing. Water can create a short to voltage in HO2S signal circuit, causing a false rich indication.
• Check HO2S 1 and MAP sensor for a faulty electrical connection to PCM.

An intermittent malfunction may be caused by a fault in HO2S 1 electrical circuit. Check wiring harness and components for an intermittent condition. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 1.8L VIBE article. Repair any electrical circuit faults found. If a DTC P0172 cannot be duplicated, information included in freeze frame data can be useful in determining vehicle operating conditions when DTC was first set.