The 2008 Chevy Express 2500 4.8L, with 200,000 miles, presents a challenge: it refuses to set the Evap, oxygen sensor, and oxygen sensor heater monitors. Despite 300 miles driven, numerous trips, and overnight cold soaks, these monitors remain unset. Interestingly, the catalyst monitor has set. This begs the question: can a catalytic converter monitor pass before oxygen sensor integrity is confirmed?
No Diagnostic Trouble Codes (DTCs) are present under engine or transmission. The battery and charging systems test fine. Fuel trim readings show a 1-6% total correction for both banks. Narrowband oxygen sensors appear neither biased nor lazy, responding as expected to propane enrichment and induced vacuum leaks. The owner, prompted by online searches, replaced Bank 1 Sensor 1 with a Denso sensor, verified against the original packaging and receipt. Comparisons with the opposite bank reveal no significant discrepancies.
Mode 6 data shows the oxygen sensors for both banks as “Complete” under “Lo and Hi voltage switch time.” However, “R/L Sensor Switch Time” and “L/R Sensor Switch Time” remain “Not Complete” on both banks. The meaning of “R/L” and “L/R” in this context is unclear.
A hard reset, involving battery terminal disconnection and subsequent reconnection, along with a PCM reset via a scan tool, followed by another 100 miles of driving, yielded no change. The Evap, O2, and HTR monitors remain unset.
Testing confirms normal functionality of the IAT, ECT, FTP, and fuel tank level sensor. The heater control duty cycle has been monitored using an Autel scanner. The Evap system, subjected to a smoke machine test, shows no leaks, and all solenoids operate correctly.
What further diagnostic steps can be taken to resolve this issue on the 08 Chevy Express? All conventional approaches seem exhausted, leaving the root cause elusive. Experienced technicians familiar with this specific platform might offer valuable insights.
