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Shane, ASE Certified Technician
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04 6.0L 2500HD check engine light flashing above 60 mph

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04 6.0L 2500HD check engine light flashing above 60 mph


a flashing engine light usually indicates a bad catalyst converter...the 0300 code is a multi cylinder may want to tune it up before you try anything else..please print




Powertrain Controls Diagnosis


Diagnostic Chart (Part 1 Of 4)
Diagnostic Chart (Part 2 Of 4)
Diagnostic Chart (Part 3 Of 4)
Diagnostic Chart (Part 4 Of 4)
The Crankshaft Position (CKP) sensor is mounted through the side of the engine block at the rear of bank 2 above the starter assembly. The CKP sensor works in conjunction with a 24X reluctor wheel on the crankshaft. The reluctor wheel is inside the engine immediately in front of the rear main bearing. The Powertrain Control Module (PCM) provides a 12 volt power supply to the CKP sensor as well as a ground and a signal circuit.

A misfire causes a change in crankshaft speed. The PCM times the interval between each pulse and compares each new time interval with the previous one in order to determine when an excessive change in crankshaft speed has occurred. You can expect a certain amount of acceleration or deceleration between each firing stroke, but if the crankshaft speed changes are more than an expected amount, the PCM interprets this as a misfire.

The PCM uses the CKP sensor for misfire detection and to control spark and fueling. As the crankshaft rotates, the reluctor wheel teeth interrupt a magnetic field produced by a magnet within the sensor. The sensors internal circuitry detects this and produces a signal which the PCM reads. The PCM uses this 24X signal in combination with the Camshaft Position (CMP) sensor 1x signal in order to accurately determine crankshaft position. The PCM also calculates a 4X signal from this information. The PCM uses the 4X signal for internal calculations. The 4X signal also provides a tach signal for any device which requires one.

Observe that as long as the PCM receives the CKP sensor 24X signal, the engine will start. The PCM can determine top dead center for all cylinders by using the CKP sensor 24X signal alone. The CMP sensor lx signal is used by the PCM to determine if the cylinder at top dead center is on the firing stroke or the exhaust stroke. The system attempts synchronization and looks for an increase in engine speed indicating the engine started. If the PCM does not detect an increase in engine speed, the PCM assumes it incorrectly synchronized to the exhaust stroke and re-syncs to the opposite cam position. A slightly longer cranking time may be a symptom of this condition.



  • No active Mass Air Flow (MAF) DTCs
  • No active Engine Coolant Temperature (ECT) DTCs
  • No active Throttle Position (TP) DTCs
  • No active Crankshaft Position (CKP) sensor DTCs
  • No active Camshaft Position (CMP) sensor DTCs
  • No active Vehicle Speed Sensor (VSS) DTCs
  • The engine speed is between 375 RPM and 5,001 RPM for automatic transmission.
  • The engine speed is between 450 RPM and 5,001 RPM for manual transmission.
  • The ignition voltage is between 10 volts and 18 volts .
  • The ECT is between -7°C (19°F) and +130°C (+266°F) .
  • Fuel level more than 10 percent
  • The TP sensor angle is steady within 1 percent .
  • The Antilock Brake System (ABS) and traction control systems are not active.
  • The transmission is not changing gears.
  • The secondary Air Injection (AIR) diagnostic test is not in progress (RPO NC1 only)
  • The A/C clutch is not changing states.
  • The PCM is not in fuel shut-off or decel fuel cut-off mode.
  • The ABS signal is not exceeding rough road thresholds.


  • The PCM determines that an emission type misfire is present.
  • The PCM determines that a catalyst damaging misfire is present.

The PCM illuminates the Malfunction Indicator Lamp (MIL) under the following conditions:

  • The PCM illuminates the MIL on the second consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed.
  • The first time the diagnostic fails, the PCM records the operating conditions in Failure Records.
  • The PCM determines the percent of misfire over a 1,000 revolution period is high enough to cause excessive fail pipe emissions. The PCM illuminates the MIL the next consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed. Or
  • The PCM flashes the MIL when the diagnostic runs and fails a catalyst damaging misfire.


IMPORTANT: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than the three ignition cycles. Review the Freeze Frame or Failure Records for the last failure conditions.

  • The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail within the same conditions that the DTC last failed.
  • A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.
  • A last test failed clears when the diagnostic runs and does not fail.
  • Use a scan tool in order to clear the MIL/DTC.


IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

  • Running the vehicle out of fuel causes sufficient misfire to set DTC P0300. A vehicle that is out of fuel may have fuel level DTCs also set.
  • Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire. If there is a misfire in Cylinder #7 it is possible that water has collected in the fuel rail.
  • If there is a misfire detected in Cylinder #4 or #6 it is possible that the fuel pressure regulator diaphragm has ruptured, causing fuel to be drawn in through the regulator vacuum line. Remove the vacuum line and inspect for fuel contamination.
  • A restricted fuel filter can cause sufficient misfire to set DTC P0300.
  • Excessive vibration from sources other than the engine could cause a misfire DTC. The following are possible sources of vibration:
  • Variable thickness brake rotor
  • Drive shaft not balanced
  • Certain rough road conditions
  • Observe, if more than one cylinder is misfiring, the scan tool may only display one cylinder misfiring. This will not be apparent until the repair is completed. Also, if an ignition coil ground circuit is open for one side of the engine, the scan tool may only display 2 or 3 cylinders misfiring. Inspect the ground circuit for the ignition coil on the cylinder bank of the engine that has more then one cylinder misfiring.

The numbers below refer to the step numbers on the diagnostic table.

  1. Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply the water. If the Misfire Current counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic table for DTC P0335 first if this condition is suspected. If a misfire is present and you suspect a fuel control problem, force the fuel system into Open Loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, A misfire may not be apparent at idle. The misfire may only occur above idle under a load. Road test the vehicle and monitor the misfire current counters. If more than one cylinder is misfiring, the misfire current counters may only increment for one cylinder. Example: Cylinders 1 and 8 are both misfiring, yet only cylinder 8 increments on the misfire current counter. If one of the injector fuses is open, only two or three misfire current counters may increment for the corresponding side of the engine.
  1. The cylinder with the more significant misfire may cause another cylinder counter to increment only by a small amount.
  1. If the engine misfire moves with the spark plug, this is good indication that you should replace the spark plug.
  1. An engine mechanical problem can cause a spark plug to gas foul. Inspect for loose rockers, collapsed lifters, or worn camshaft lobes.
  1. If the customer concern is the MIL flashing, this indicates that a Catalyst Misfire has occurred. Drive the vehicle in the conditions to run the catalyst diagnostic.



Customer: replied 7 years ago.
Tune up has already been done, I don't think your answer will fix the issue. It doesn't explain why I am getting 2 MPG less- which tells me the engine thinks it's lean so it's adding fuel. If it was a crank sensor I would feel a miss, and I can tell you that if the light wasn't flashing and the mileage hadn't dropped you wouldn't even know something wasn't right. Still have great throttle response, it does it less while towing. Before light starts flashing O2 front sensors go to 13.4 and stay there until light stops flashing. Does the additional info help?


allot of alcohol in the fuel would cause the light to flash and you would get a lot less mpg's.....also,as i said before a bad catalyst would cause all the problems you just gave..


Customer: replied 7 years ago.
I agree with both of those statements, but this condition has been going on and getting worse for 4 months. I have filled up at stations throughout the state so the alcohol/ethanol content in the fuel should not be an issue. If it was a bad converter I would think it would happen at speeds below 60 MPH (and towing would make it more noticable). Usually when you have this condition it will either stink, or plug your exhaust. Even while light is flashing, I can go to WOT and the truck pulls like crazy. Is there a way to prove your theory without replacing the converter?


a muffler shop may be able to check the converter with a sniffer...another thing you may want to look at is the fuel pressure,,a weak fuel pump may also cause this,,these must have no less then 60psi at all times


Customer: replied 7 years ago.
Thanks for your help, but if you haven't fixed this before on multiple other trucks with the 6.0L than you don't have the experience I need with this specific issue. Chevy has an issue here and I think they are not letting the cat out of the bag with the issues and remedies. FYI, same for their speedo issues on most 04' models which require a new dash.
Hey just a shot in the dark, try to relearn the crank sensor to the pcm via scan tool(may need tech2)I've had this fix a couple with out replacing parts.
Customer: replied 7 years ago.
Thanks for taking a shot, have you ever had that not work? Also, since you've worked for the manufacturer: is there a PCM issue (replacement or relearn) that the dealer needs to do for this issue? Warrenty specifications? Or is it a crank sensor every time?
I have had two occasions with a G van but it had a 6.0 liter in it the engine light would come on highway speeds 60-70mph with a P0300 and no noticeable misfire or performance issues. I would relearn the crank sensor variation and road test the vehicle with no light. Sounds like similar concern. No TSB's or warranty for the issue. I had the first van a few days before I tried this.
Customer: replied 7 years ago.

Your answer was a little hard to interpret. So you had two occasions where this didn't work? My vehicle (6.0L) is having the 60-70MPH flashing check engine light with a P0300 with no noticable misfire or performance issues. Would it make more sense to relearn, or replace the sensor if that's the issue? I think the sensor cost is about $65, vs dealer time which would be more than that. When installing the new sensor do you need a Tech2 to sync anything or is it just plug and play?

I had two occasions with this same concern, relearning the crank sensor fixed the issue. However if you replace the sensor(I never have replaced them) you will need to relearn the sensor anyway.
Shane, ASE Certified Technician
Category: Chevy
Satisfied Customers: 173
Shane and 4 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
If it works I will give you credit for the answer.
fair enough
Did you say the oxygen sensor went to BATTERY voltage and then started flashing the check engine light ?
Customer: replied 7 years ago.
It's not battery voltage, but short term fuel trim went to 13.4 every time right before the check engine light started flashing.
I have not run into this particular problem but , I was skimming through and saw the 13.4 and thought that it was ready battery voltage . If the heated o2 is showing battery volts ( shorted internally ) the computer is dumping excess fuel . That would account for the mpg drop and a flashing check engine light is means that if you continue driving it will damage the catalytic converter , not that it is a failed cat .
When you get this truck figured out, will you post the fix. I'm just curious on what's causing this.
Customer: replied 7 years ago.
Yes, I will post. I have yet to try the crank sensor relearn because I am waiting for a Tech2 to become available. We also tried the SB about the bad PCM ground on the right head. Mine wasn't corroded, but we performed anyway and nothing changed. Might be next week before this gets done, but I have saved this info and will get back to you.

Here is the service bulletin ...On this concern

Subject: 97-05 Vehicles with 4.8 5.3 5.7 and 6.0 Gen III V8 Engines and Misfires on One Bank - kw CEL diagnostics driveability DTC P0171 P0172 P0174 P0175 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 #PIP3056 - (07/12/2004)

Models: (00-05 Cadillac Escalade - C6 and K6) and (02-05 Chevrolet Avalanche - C1, C2 K1 and K2) and (99-05 Chevrolet Silverado - C1, C2 K1 and K2) and (00-05 Chevrolet Suburban - C1, C2, K1 and K2) and (00-05 Chevrolet Tahoe - C1 and K1) and (99-05 GMC Sierra C1, C2 K1 and K2) and (00-05 GMC Yukon - C1, C2, K1 and K2) and (03-05 Chevrolet Express G1, G2, G3, H1, H2 ) and (03-05 GMC Savana G1, G2, G3, H1, H2 ) and (04-05 Buick Rainier S1, T1 ) and (03-05 Chevrolet Trail Blazer EXT S1, T1 ) and (03-05 GMC Envoy XL XUV S1, T1 ) and (03-05 Hummer H2) and (04-05 Cadillac CTS-V DB) and (98-02 Chevrolet Camaro FB) and (97-04 Chevrolet Corvette YB) and (98-02 Pontiac Firebird FB) and (04-05 Pontiac GTO VB)

The following diagnosis might be helpful if the vehicle exhibits the described symptoms.

Check Engine Light with a P0300 due to 2 - 4 cylinders misfiring on the same bank of the engine. DTCs P0171, P0172, P0174 or P0175 may also set for the misfiring bank.

If the published misfire diagnostic does not isolate the cause, perform the following suggestions as necessary:

  • Perform a fuel injector balance test for all 8 cylinders. If a fuel injector concern exists, it is possible to misfuel an entire bank of the engine, causing multiple cylinders on the same bank to misfire even though the root cause is a single fuel injector.
  • Inspect O2 sensor connections on the misfiring bank for corrosion or water intrusion. If water intrusion is found on the right bank, it may be due to the AC Evaporator Condensation dripping onto the O2 sensor harness. If this condition is found, reposition and shield the harness to prevent a repeat concern and repair the connections.
  • Check for excessive exhaust backpressure using the restricted exhaust diagnosis.
  • Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If so, replace the O2 sensor.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

NOTE: GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition.

Customer: replied 7 years ago.
Thanks for the input, although I don't think this Bulletin apply's to this situation. The engine is NOT missing, but the computer thinks it is. A injector balence test may help diagnose the issue, but the other items have already been ruled out. Still waiting on a Tech2.
I thinks its a injector issue. If you have a scanner with livce data i would watch for is most likley one bad injector or a converter flow issue
Customer: replied 7 years ago.
It could be an injector issue, but then why would both front O2 sensors go to 13.4 on the fuel trim at the same time right before the light starts blinking? It would have to be on one bank or the other if it was a bad injector. Plus a injector would act up at multiple RPM's and throttle conditions. This only happens at a specific MPH range even when towing.
just curious if you got this one figured out??
Customer: replied 7 years ago.
Shane, thanks for checking back. This afternoon I got a hold of a top of the line scanner. Misfire history on Cyl #1, #5, #6, #8. 1 & 5 being the worst. Swapped out coil's, wires, and plugs with the known good cyl's. No change. Computer is telling me these cyl's are missing like crazy but even with multiple Tech's nobody can detect a misfire is actually happening. You can duplicate this issue on the scanner without a road test above 2500 RPM. Cyl's 2,3,4,7 have absolutely no misfires being detected. We did a crank sensor relearn (this won't happen with a HyperTech installed so I uninstalled it) did not affect this issue. At this point I believe this is a bad PCM, but I've suspected this all along since all the issues and data don't pinpoint the issue. I need to contact the dealer to see what they want for a new PCM. I'll keep checking the thread to see if anyone has any other info and post back when it's fixed.
Customer: replied 7 years ago.
Shane, got more info. Had a friend at the dealer reprogram PCM, said many updates got installed. After this was complete computer set a P0300 code on road test, this code would always clear itself before coming to a stop before. Check engine light didn't need 60 MPH or 2500 RPM to flash anymore, would flash above 1700 RPM now. Check engine light on solid when not flashing. Had previously eliminated ignition issue by swapping coil, plug, and wire between known good cyl and one with all the misfire history. Did injector balence test, all had 16 PSI drop, swapped injectors on good cyl with bad. Misfire data still present. Seen slow B1S2 O2, so I disconnected both front bank O2's. Misfire data still present. Thought about your post and was trying to think of what else could be telling the computer it was missing when it wasn't really missing. Had to do Crank relearn 4 times before it took, but when it took all misfire data went away.XXXXXtest good. Please respond to this so I can accept your answer. Thanks!!!
I'm glad to hear I could help!