P0420Catalyst System Efficiency Below Threshold (Bank 1)P0430Catalyst System Efficiency Below Threshold (Bank 2)
Determining catalyst efficiency/switch ratio: The upstream HO2S sensors will have a high switch frequency, due to normal closed loop fuel control. With an efficient catalyst, the downstream HO2S will have a low switch frequency. The switch ratio is determined by dividing the number of downstream switches by the number of upstream switches over a given period of time. As the catalyst ages (or if the catalyst is damaged or contaminated), the downstream switches will increase. When the downstream switch rate crosses a threshold value (approximately 0.75 switch ratio), a code is stored (P0420 and/or P0430) and the MIL illuminates
p0171 1nd p0174
On a fully warmed up engine, look at Long Term Fuel Trim at idle, in Neutral, A/C off, (LONGFT1 and/or LONGFT2 PIDs). If it is more negative than -12%, the fuel system has learned lean corrections which may be due to the MAF sensor over-estimating air flow at idle. Note that both Banks 1 and 2 will exhibit negative corrections for 2-bank system. If only one bank of a 2-bank system has negative corrections, the MAF sensor is probably not contaminated.
On a fully warmed up engine, look at MAF voltage at idle, in Neutral, A/C off (MAF V PID). If it's 30% greater than the nominal MAF V voltage listed in the Powertrain Control/Emissions Diagnosis (PC/ED) Diagnostic Value Reference Charts for your vehicle, or greater than 1.1 volts as a rough guide, the MAF sensor is over-estimating air flow at idle.
If at least two of the previous three steps are true, proceed to disconnect the MAF sensor connector. This puts the vehicle into Failure Mode and Effects Management (FMEM). In FMEM mode, air flow is inferred by using rpm and throttle position instead of reading the MAF sensor. (In addition, the BARO value is reset to a base/unlearned value.) If the lean driveability symptoms go away, the MAF sensor is probably contaminated and should be replaced or cleaned