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How do you set up flex fuel so the car adjusts commanded lambda between E85 and pump gas?

Install a GM flex fuel sensor in the fuel line and wire its signal to the ECM — P59: pin 56 on the blue connector (signal, +12V, ground); E38: add pin 40 to the J1 (73-pin) connector with a Molex MX64 terminal. Then enable Flex Fuel in the tune, set the stoich AFR vs. alcohol table from a factory flex-fuel file (default 38%), and enable the P0178/P0179 sensor DTCs. The ECM reads ethanol as 50-150 Hz (0-100%) and blends commanded lambda between gasoline stoich (~14.7) and E85 stoich (~9.8) automatically.

Last updated October 28, 2026

Who this applies to

Computers: P59, E38, E67
Engines: LM7, LQ4, LQ9, LS1, LS2, LS3
Transmissions: all
Years: 2003-2013
Not for: P01 512KB PCMs (flex tables not present)

Gen III P59 (1MB PCM), HP Tuners

Wire the sensor signal to pin 56 on the blue connector (A to pin 56, B to +12V, C to ground) pin 56, blue connector
Engine > Fuel > Open & Closed Loop > Flex Fuel > Flex Fuel: ENABLE
Set Flex Fuel Default to 38% (copied from a factory flex-fuel Tahoe file)
Engine > Fuel > General > Air Fuel Ratio > Stoich AFR: copy the table from the factory flex-fuel file
Enable DTCs P0178/P0179, set to MIL on second error 1-MIL on second error

Gen IV E38, HP Tuners

Disconnect the battery, then add a pin at J1 (73-pin) connector position 40 — the flex fuel frequency input J1 pin 40
Use a female Molex MX64 pin (Mouser 538-33467-0005) and 20 or 22 gauge wire
Wire the sensor signal to pin 40; power and ground to the sensor
Engine > Fuel > General: enable Flex Fuel and Flex Fuel Sensor; set the min/max and volume values if blank; enable the flex fuel sensor DTCs

Verifying the sensor works

Log Alcohol % / ethanol content in VCM Scanner
Fill with E0, note the reading (~0-1%), add a few gallons of E10 and watch the percent rise — if it rises, the whole chain works
Expect pump E85 to read anywhere from E55 to E78 — blends vary by station and season E55-E78

Careful

E85 needs ~42% more flow — size injectors and pump for it; a stock fuel system will run out.
Don't hard-code stoich to 9.765 for pump fuel — pump E85 varies (E55-E78); that's what the sensor is for.

More in fuel, E85 & boost, or build a checklist for your swap with the tune checklist tool.

Go deeper

Check this first

Log alcohol % with known E0/E10 in the tank — no change means the sensor or wiring is dead
Check P0178/P0179 — a sensor circuit code means wiring or power, not the tune

Try this, then this

Verify sensor power, ground, and the signal pin first — Most flex failures are wiring, not the tune. Worked if: Scanner shows a live Hz reading
Confirm Flex Fuel is enabled and the sensor DTCs are enabled (not No Error Reported) — The ECM ignores the input if the sensor diagnostics are turned off. Worked if: Alcohol % reads a sane value
Test with E0 then E10 and watch the percent rise — Proves the whole chain — sensor, wiring, ECM, tune — end to end. Worked if: Percent rises with E10

How you know it's fixed

Alcohol % tracks the fuel you pour in (E0 reads ~0-1%, E10 reads ~10%)
Commanded lambda swings with the blend
No P0178/P0179 codes

Key numbers

What	Value
P59 flex signal pin	pin 56, blue connector
P59 flex sensor power/ground	B to +12V, C to ground
Flex default from factory FF file	38%
E38 flex input	J1 connector pin 40 (73-pin)
E38 connector terminal	Molex MX64 female, Mouser 538-33467-0005
E85 fuel flow vs gasoline	~42% greater
P01 flex support	not present on 512KB P01

Every option we know of

We may earn a commission when you buy through links on this page.

Maker / product	Part #	Fits	Where	Status / notes
Chevrolet Performance Flex Fuel Sensor, short inlets (3/8 in)	13577379	Universal GM flex fuel	Buy briantooleyracing.com	In production
Burger Motorsports (Fuel-It) Genuine GM Ethanol Sensor	13577429 (standard) / 13577379 (short)	Universal	Buy burgertuning.com	In production Needs an ethanol analyzer to display content on non-GM dashes
DSX Tuning Flex fuel hardware with PCM-connector terminal/wire		E38	dsxtuning.com	Small batch

Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

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