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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8672
Experience:  I am ASE Master Truck Certified.
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97pete 379 model 550cat having trouble under load pulling

Customer Question

97pete 379 model 550cat having trouble under load pulling hills. It bucking bad. Changed both fuel filters. Every now and again it grows code 21. Help
Submitted: 7 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 7 months ago.
Thanks for asking your question. I will try and help you out.
Does the yellow check engine light come on when this happens?
Customer: replied 7 months ago.
Customer: replied 7 months ago.
Then it goes out returns to code 55. Runs great for a while. I've cleaned battery cables
Expert:  Cat Man replied 7 months ago.
Code 21 indicates a fault with either the 5 volt sensor circuit or the 8 volt throttle position sensor.
This code can be caused by a shorted boost pressure sensor, atmospheric pressure sensor, oil pressure sensor, or TPS sensor.
Since the engine bucks I would more suspect the boost sensor or TPS. Unfortunately to verify a service tool will be needed to read the ECM.
Please reply back if you need more help or have more questions. I am happy to assist.
Expert:  Cat Man replied 7 months ago.
This code is normally caused by a shorted 5 volt sensor. All three pressure sensors use the same 5 volt supply from the ECM. When there is a short, all the sensors do not report to the ECM. It could also be a short in the wiring harness. It's possible the short is in the ECM but that's very rare.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 7 months ago.
Anything I can do on side of road
Expert:  Cat Man replied 7 months ago.
The only thing would be to unplug the oil pressure and atmospheric pressure sensors and try to drive. If one of those are the problem, you should not lose power but only have a check engine light on. If you still have the problem. I would suspect a faulty boost pressure sensor, harness, or TPS fault.
Please reply back if you need more help or have more questions. I am happy to assist.

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