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Mike S.
Mike S., Chevy Mechanic
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Experience:  ASE Certified Master Technician
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my 1997 tahoes codes come up po300/po138/po161

Resolved Question:

my 1997 tahoe's codes come up po300/po138/po161
Submitted: 7 years ago.
Category: Chevy
Expert:  Mike S. replied 7 years ago.
New plugs and new wires and you still have a P0300 random misfire. Did you clear the codes after the new plugs and wires? If so maybe a vacuum leak, let me take a look at the manual and also look up the other 2 codes. BRB
Expert:  Mike S. replied 7 years ago.

2wd or 4wd. The 2wd 8 Cylinders 5.7L is a SFI (sequential fuel injection) system. Clogged fuel injectors are a possibility. If you can hear all the injectors clicking even if having to use a stethoscope or long screwdriver in your fist up to your ear confirms that the system is at least functioning electrical wise. If they all click but some engines missfire even with new plugs and wires then there may be some fuel injectors clogged. A lean condition would result and possibly throw off the O2 sensors and set a too lean code or O2 code. Let me look up those other codes now.

Just as I thought, Trouble Code: P0138 (5.7L V8 VIN R Auto) HO2S Circuit High Voltage Bank 1 Sensor 2, Trouble Code: P0161 (5.7L V8 VIN R Auto) HO2S-22 (Bank 2 Sensor 2) Heater Circuit Malfunction.

Trouble Code: P0300 (5.7L V8 VIN R Auto).

Give me a few minutes, I see a few TSB's I want to upload you, plus the 3 code explanations, causes and testing procedures and repair procedures.

Engine Misfire Detected

Expert:  Mike S. replied 7 years ago.

If the VCM evaluates 3 or more cylinders are misfiring, then this determines that a random misfire has occurred. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

Many different condition could cause an intermittent misfire.

Check for contaminated and a low fuel level and the following conditions:

Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.

If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system.

Make repairs as indicated by the Fuel System Diagnosis. Refer to Fuel System Diagnosis.

When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. The pump is designed to provide fuel pressure (60-66 PSI), above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure.


Check fuel pressure, check for a partially disconnected fuel pulse dampener (Pulsator). Check for a restricted fuel lines.


P0138 is actually the Heated Oxygen Sensor Circuit High Voltage and/or a too rich mixture. Check the fuel pressure: If the pressure is too high, the system will run rich. P0138 is bank 1 of driver's side running too rich. This is most likely a leaking injector the other most likely problems that occur with fuel injectors. May be due to dirt picked up in the tank and getting past the fuel filter and/or picked up in the air and getting past the air filter. More possible causes of a rich condition could be a clogged pcv system and/or a dirty air filter. Also a full EVAP canister can cause a rich condition as well as a leaking fuel pressure regulator diaphragm. Check the vacuum line to the regulator for fuel.

More causes include the TP sensor: An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving or the HO2S is internally shorted.


P0161 is the Heated Oxygen Sensor Malfunctioning on bank 2 or the passenger side. Possible causes include a poor connection or a damaged harness, the HO2S heater is mal-functioning or the ignition feed circuit to the HO2S is not open or shorted. Another cause is an open or shorted HO2S heater element.


O2 sensors are quite expensive or I would just recommend replacing the Heated Oxygen Sensor #2 on the passenger side, check the fuel pump pressure, replace the fuel filter and cleaning the fuel injectors. Since the cost of the sensor may be over $50, I would test that 1 according to the test procedures in the Chilton manual. The other sensor is probably just reporting on the rich condition and fuel system problem and possible leaking injectors, dirty fuel or too high a pressure. Most likely the pressure isn't too high unless of a kink in the line, so I think a new fuel filter if not done lately, test the passenger side O2 sensor and clean or replace the fuel injectors that are causing the misfire. You can perform the fuel injector balance test but you may have to take off the upper intake in order to disconnect the fuel injector wires to do a cylinder balance test or because of the new spark plugs and wires just trust a regular cylinder balance test.


First of all you will need a 12-volt test light and about 8-12 inches of neoprene vacuum hose. The vacuum hose will conduct electricity cause it is carbon based, use an ohmmeter if you aren't sure and see if the vacuum hose you have will conduct.

Now, cut off the same # XXXXX cylinders you have in small equal lengths of the vacuum hose. 2-3 inches will do fine. The small diameter kind like to carburetors, etc will work as long as they fit over the distributor cap tower connection.

Now, mark all your spark plug wires at the cap or coil pack and remove them all. Put those short pieces of vacuum hose on the distributor cap or coil pack connections and shove the other end of the vacuum hose into the spark plug wire boot until it makes a good connection.

Now connect your 12 volt test ground clip to a ground and start the vehicle. With the engine running touch the 12 volt test light to each of the vacuum hose 1 at a time and listen for the cylinder to short out and die and drop in r.p.m. They should all be about equal. If 1 or a few don't drop or do anything than you have your dead cylinder there.


Also, I see the use of E85 fuels can cause a misfire and also Catalytic Converter Damage Due to Installation of Alarm Systems.


For the explanations, causes and repair procedures for P0300 P0138 P0161, click here.


For the Fuel System Diagnosis, click here.

For the TSB's on Usage of E85 Fuels and Catalytic Converter Damage Due to Installation of Alarm Systems, click here and here.


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