The cure is to clean the injectors and valves. Cleaning should be recommended for any engine that is suffering a performance complaint or has more than 50,000 miles on the odometer. Cleaning the throttle body can also help eliminate idle and stalling problems that plague many of today's engines.

The O2 sensor is the master switch in the fuel control feedback loop. The sensor monitors the amount of unburned oxygen in the exhaust and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich). The computer uses the O2 sensor's signal to constantly fine tune and flip-flop the fuel mixture so the catalytic converter can do its job and clean the exhaust. If the O2 sensor circuit opens, shorts or goes out of range, it usually sets a fault code and illuminates the Check Engine or Malfunction Indicator Lamp. But many an O2 sensor that is badly degraded will continue to function well enough not to set a fault code but not well enough to prevent an increase in emissions and fuel consumption. So the absence of a fault code or warning lamp doesn't mean the O2 sensor is doing its job.

Deterioration of the O2 sensor can be caused by a variety of substances that find their way into the exhaust (such as lead, silicone, sulfur, even oil ash) as well as environmental factors such as water, splash from road salt, oil and dirt.

A sluggish sensor may not allow the computer to flip-flop the fuel mixture fast enough to keep emissions within acceptable limits. A dead sensor will causes the system to go back into open loop with a fixed, rich fuel mixture. Fuel consumption and emissions go up, and the converter may suffer damage if it overheats.

The best way to check O2 sensor performance is with a digital oscilloscope. A good sensor should produce an oscillating waveform that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2500 rpm).

When the mixture is made artificially rich by injecting some propane into the intake manifold, the sensor should respond almost immediately (within 100 milliseconds) and go to the maximum (0.9v) reading. Likewise, making the mixture artificially lean by opening a vacuum line should cause the sensor's output to drop immediately to the minimum (0.1v) reading.

• Spark plugs (gapped to the correct specs, of course). Consider long life plugs on applications where plug accessibility is difficult or where longer service life may be beneficial
• Rotor and/or distributor cap (if required)
• Fuel filter; Air filter; PCV valve and breather filter
• Other parts on an "as needed" basis (things like spark plug wires, belts, hoses, fluids, etc.)
• Check and adjust (if required on older vehicles) ignition timing, idle speed and idle mixture; O2 sensor(s).

• Battery voltage (very important with all of today's onboard electronics).
• Charging voltage
• Power balance or dynamic compression (to identify any mechanical problems such as leaky exhaust valves, worn rings, bad head gasket, bad cam, etc. that could adversely affect compression and engine performance)
• Engine vacuum (to detect air leaks as well as exhaust restrictions)
• Operation of the fuel feedback control loop (to confirm that the system goes into closed loop operation when the engine warms up)
• Scan for fault codes (to verify no fault codes are present, or to retrieve any codes that may be present so they can be diagnosed and eliminated)
• Check exhaust emissions (this should be a must in any area that has an emissions testing program to confirm the vehicle's ability to meet the applicable clean air standards, and to detect gross fuel, ignition or emission problems that require attention)
• Verify idle speed (should be checked even if computer controlled to detect possible ISC motor problems); Idle mixture (older carbureted engines only, but injector dwell can be checked on newer vehicles to confirm proper feedback fuel control)
• Check ignition timing -- if possible (should be checked even if it is not adjustable to detect possible computer or sensor problems)
• Operation of the EGR valve.