How Does The PCM Know A Rich Condition Exists?
What will help you to diagnose these two codes, is knowing how the PCM sets them. I won't go into any complicated automotive theory, just the basic facts that you need to know:
- 1
When the engine is running, the PCM is constantly adjusting the amount of fuel it's injecting.
These adjustments are called Short Term (ST) Fuel Trim and Long Term (LT) Fuel Trim. And you and I are able to see the values that are generated on a scan tool with Live Data capability.
Each engine bank is adjusted separately from one another. For this reason, you'll see a ST FTRM 1 and LT FTRM 1 for bank1 and ST FTRM 2 and LT FTRM 2 for bank 2.
This fine-tuning (of the amount of fuel injected) is accomplished mainly via the oxygen (O2) sensors, although every sensor on the engine also plays an important part in adjusting the fuel trim of both engine banks. - 2
The oxygen sensors that have a direct bearing on how the PCM fine tunes the fuel injection are O2S11 and O2S21.
O2S11 stands for Oxygen Sensor Bank 1 Sensor 1. This bad boy is located on the exhaust pipe that connects to the bank that houses cylinders #1, #2, #3, and #4. Also, this O2 sensor is upstream from the catalytic converter.
O2S21 stands for Oxygen Sensor Bank 2 Sensor 1. This bad boy is located on the exhaust pipe that connects to the bank that houses cylinders #5, #6, #7, and #8. Also, this O2 sensor is upstream from the catalytic converter.
- 3
When the PCM injects too much fuel (causing the air fuel mixture to turn rich), the oxygen sensor produces a voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.
When the PCM sees this rich condition, it starts to inject less.
When the PCM starts to cut fuel, the Short Term (ST) Fuel Trim values dive down into a negative value. This negative value lets you and me know that it's cutting fuel. - 4
As the PCM starts to cut back fuel (causing the air fuel mixture to turn Lean), it may go too far and not inject enough. Here again the O2 sensor saves the day by reporting a voltage that can go as low as 0.050 to 0.100 Volts.
Voltages in this range let the PCM know that the air/fuel mixture is too Lean and starts to inject more.
When the PCM starts to add more fuel, the Short Term (ST) Fuel Trim values to go into a positive value to let you and me know that more fuel is being added. - 5
If all is peachy, the oxygen sensors will switch between a lean and rich condition several times every few seconds the whole time the engine is in operation.
Also, the fuel trim values will move up and down between a positive and a negative value. Usually no more than 10% and no less than -10%.
- 6
If the O2 sensor stays stuck at a rich condition (anything above 0.500 millivolts) for too long, the PCM will try to reduce the amount of fuel to change the O2 sensors values.
Since the PCM is reducing fuel, the LT Fuel Trim (Long Term Fuel Trim) values will go into their maximum negative value of -20% (the Long Fuel Trim values are the ones that let us know what's happening over an extended period of time. In contrast, the Short Term Fuel Trims reflect immediate adjustments to the A/F mixture).
If this rich condition continues without switching to a lean condition for too long, the PCM lights up the check engine light and reports a P0172 or a P0175 trouble code (depending on the bank that's being affected).
OK, let's get testing in the next page.
