OBD-II trouble code
P0179: Fuel Composition Sensor Circuit High
SAE J2012 APR2002: fuel composition sensor circuit high. On the 2005 Chevrolet Tahoe 5.3L VIN Z chart, P0179 sets when fuel-composition frequency is more than 155 Hz for 12.5 seconds. That includes the 170 Hz contamination or internal-fault output. Unplugging the sensor does not locate an open.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $130 – $180 (check engine light diagnosis and testing, labor only; composition-sensor replacement is not priced in this pair)
- DIY difficulty
- Advanced DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0179 mean?
SAE J2012 APR2002 names P0179 Fuel Composition Sensor Circuit High. On the 2005 Chevrolet Tahoe 5.3L VIN Z, the chart descriptor is Fuel Composition Sensor Circuit High Frequency. The code sets when fuel-composition frequency is more than 155 Hz for 12.5 seconds, after engine run time of more than 30 seconds, with system voltage greater than 10.9 volts. On that sensor 150 Hz is 100 percent ethanol and it is still a square wave, and it is not above 155 Hz. The same chart says 170 Hz means contaminated fuel or an internal sensor fault, and 170 Hz is above 155 Hz, so that output is this code on this chart. A DC voltmeter hides the frequency.
That Tahoe text gives the sensor 12-volt power and a chassis ground, and a 5-volt pull-up inside the PCM. The sensor pulls that pull-up to ground in pulses. With the sensor unplugged, the PCM pull-up stays high whether or not the signal wire is intact, so that result does not locate an open. The chart does not call the harness open from an unplug. Its signal test reads Fuel Comp. Sensor Frequency at idle. Above 155 Hz, compare the scan frequency with the J 44175 fuel composition tester: a difference under 12 Hz goes to fuel-system cleaning, and a difference of 12 Hz or more goes to sensor replacement. If idle is not above 155 Hz, the chart agitates the fuel by pushing the rear of the vehicle from side to side for 5 seconds and watches the parameter for 5 minutes. A rise above 155 Hz goes to cleaning. No rise goes to the intermittent test. Check continuity of the signal circuit back to the module before calling the harness open.
GMC Mitchell document 5297761 keeps 50 Hz at 0 percent and 150 Hz at 100 percent, and it puts a 12-volt pull-up on the signal from the fuel pump driver module. Do not copy the Tahoe 155 Hz set point, the J 44175 comparison, or the 5-volt pull-up onto that document. Use the diagram.
The sensor in the Tahoe description is inline. Depressurize before the fuel connections are opened. Those quick-connects are different diameters. No sourced parts-and-labor price for the sensor, or for the chart's fuel-system cleaning, was found. The pair on this page is diagnosis labor.
Common causes
- Frequency above 155 Hz for 12.5 seconds on the 2005 Tahoe chart
- 170 Hz contamination or an internal sensor fault, which that chart places above the 155 Hz set point
- Scan frequency and the J 44175 tester within 12 Hz, which that chart sends to fuel-system cleaning
- Scan frequency and the J 44175 tester 12 Hz or more apart, which that chart sends to sensor replacement
- A harness called open because the PCM 5-volt pull-up stayed high with the sensor unplugged
- The Tahoe 155 Hz set point copied onto document 5297761, which uses a 12-volt pull-up from the pump driver module
Symptoms
- P0179 stored
- Fuel Comp. Sensor Frequency above 155 Hz on the Tahoe chart
- 170 Hz, the contamination or internal-fault frequency in that Tahoe text
- Malfunction indicator lamp on the second consecutive ignition cycle that the Tahoe diagnostic runs and fails
- Freeze frame and failure records stored by that chart
- P0178 on that Tahoe chart when frequency is below 45 Hz, which is the low-frequency code rather than this one
Diagnostic steps
- 1.Confirm the diagram shows a composition sensor and which module provides the pull-up. The Tahoe chart uses the PCM 5-volt pull-up. Document 5297761 uses 12 volts from the pump driver module.
- 2.On the 2005 Tahoe chart, idle and read Fuel Comp. Sensor Frequency. Above 155 Hz is the set point, after more than 30 seconds of run time and with system voltage above 10.9 volts. 150 Hz is 100 percent ethanol on that scale and is not above the set point. 170 Hz is above it.
- 3.If idle frequency is above 155 Hz, compare it with the J 44175 tester. A difference under 12 Hz: the chart cleans the fuel system. A difference of 12 Hz or more: the chart replaces the sensor.
- 4.If idle frequency is not above 155 Hz, push the rear of the vehicle from side to side for 5 seconds, as that chart says, and watch the parameter for 5 minutes. A rise above 155 Hz goes to cleaning. No rise goes to the intermittent test.
- 5.On that Tahoe sensor the PCM 5-volt pull-up stays high with the sensor unplugged whether or not the harness is intact, so that does not locate an open. Check continuity of the signal circuit back to the module, or use the frequency test on that chart, before calling the harness open. Do not open fuel fittings for the electrical test.
- 6.Depressurize before any fuel connection is separated. On the Tahoe sensor, refit the quick-connects to the diameters that description assigns.
- 7.Do not transfer the Tahoe 155 Hz set point, the 170 Hz call, or the J 44175 comparison onto a vehicle that is not using that chart.
- 8.Verify frequency against the fuel in the tank after the repair.
Repair cost
$130 – $180 (check engine light diagnosis and testing, labor only; composition-sensor replacement is not priced in this pair)
RepairPal check engine light diagnosis, 3 September 2026, is $125–$183 labor. Rounded, the printed pair is $130–$180. It is diagnosis only. No parts-and-labor total for the inline composition sensor was found on RepairPal or YourMechanic. Fuel connections stay closed until the published depressurizing procedure has been followed.
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DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.