P0688: Engine computer power relay sense circuit fault

P0688 ECM/PCM power relay sense circuit open (auto shutdown relay)

Severity: Inspect soon

The code can cause hard starting, stalling or a no-start, so it needs a check soon even if the car runs now.

It gets more serious if:

Stop: Stop driving if the engine stalls or cuts out while moving. The source describes stalling and a no-start as the risk with this code.

P0688 in short

P0688 means the engine computer did not see the voltage it expects from its own power relay (on Chrysler, Dodge, Jeep and Ram this is the auto shutdown relay). A weak battery, a bad relay or fuse, or a corroded connector is most likely. It can cause stalling or no-start.

The engine computer powers itself and the fuel and ignition parts through a relay. A separate sense wire tells the computer whether the relay output has voltage. If it does not see the expected voltage, it sets P0688.

That points first to the battery, the relay, its fuses and the wiring and connectors, all cheap and easy checks. An internal computer fault is possible but the source says to rule out the others before replacing it.

On 2019-2022 Ram 1500, an FCA bulletin lists P0688 (auto shutdown relay sense circuit low, found in the PCM) in a no-crank or crank no-start condition. On Chrysler models, a bulletin tells technicians to contact FCA before replacing a TIPM when P0688 or P0685 is active.

Most likely causes

Cause 1: Weak battery or low cranking voltage
Common general cause
Why it ranks here
The source lists a weak battery and low cranking voltage first, and they are the easiest to test.
What would change this
Rises if the battery fails a load test or voltage drops while cranking. Falls if the battery is strong.
Cause 2: Faulty power relay or blown ECM fuse
Common general cause
Why it ranks here
The source names a defective relay and a blown ECM fuse as direct causes of the missing sense voltage.
What would change this
Rises if the relay does not click or the fuse is open. Falls if both test good.
How to test the engine controls ignition relay
Cause 3: Loose or corroded relay harness connections
Common general cause
Why it ranks here
The source lists loose relay harness connections and corroded connections among the common causes.
What would change this
Rises if terminals are green, burnt or loose. Falls if the connections are clean and tight.
How to test the underhood fuse block (engine compartment fuse box)
Cause 4: Auto shutdown relay sense fault in the PCM (2019-2022 Ram 1500)
Supported by vehicle data
Why it ranks here
FCA bulletin S1918000013 (and Rev. A) lists P0688 for a no-crank or crank no-start on the 2019-2022 Ram 1500. Only the NHTSA summary was read, so the exact repair is not stated here.
What would change this
Rises on an affected Ram 1500 with a no-start and P0688. Falls on other vehicles.
How to test the engine control module (ecm / pcm)
Cause 5: Faulty engine computer or TIPM
Possible — needs testing
Why it ranks here
The source lists a faulty computer, but says to rule out the other causes first. The Chrysler bulletin 68232879A also advises contacting FCA before replacing a TIPM for P0688.
What would change this
Rises only after battery, relay, fuses and wiring test good. Falls if any earlier cause is found.
How to test the engine control module (ecm / pcm)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

Step 1: Check battery voltage at rest and while cranking, and clean and tighten the battery cables.
Low risk
- IfVoltage is low or drops sharply when cranking
  It meansCharge or replace the battery first.
  Points to: Weak battery or low cranking voltage
- IfBattery and cables are good
  It meansCheck the relay and fuses.
  Go to step 2
Step 2: With the key off, inspect the engine computer power relay and its fuses, and swap in a known-good relay of the same type if the vehicle allows.
Low risk
- IfThe relay is bad or a fuse is blown
  It meansReplace it and find out why the fuse blew.
  Points to: Faulty power relay or blown ECM fuse
- IfRelay and fuses test good
  It meansCheck connectors and bulletins.
  Go to step 3
Step 3: Inspect the relay and computer connectors for corrosion or loose terminals, and ask a dealer about a bulletin for P0688 (Ram 1500 2019-2022, Chrysler TIPM).
Use cautionTool: multimeter
Warning:Disconnect the battery before unplugging computer or fuse block connectors.
- IfTerminals are corroded or loose
  It meansRepair the connection.
  Points to: Loose or corroded relay harness connections
- IfA bulletin applies
  It meansFollow it before replacing parts.
  Points to: Auto shutdown relay sense fault in the PCM (2019-2022 Ram 1500)
- IfEverything checks out
  It meansThe computer may be at fault; have a shop confirm.
  Points to: Faulty engine computer or TIPM

Symptoms

Engine cranks but does not start, or does not crank
Hard starting or intermittent stalling Gets more serious: stop and have it checked.
Check engine light

How to fix

Step 1: Check the battery terminals and cables for corrosion and tightness.
Step 2: Check the engine computer fuses and relay.
Warning:Replace a fuse only with the same rating, and find out why it blew.

Parts and cost context

Engine controls ignition relay:A relay and fuse are inexpensive. Rule them out before replacing the computer. Labor varies; get a quote.
Engine control module (ECM / PCM):Replacement needs programming; confirm other causes first.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Ram

2019–2022 · 1500
Supported by vehicle data
FCA bulletin S1918000013 (and Rev. A) covers a no-start with P0688 stored in the PCM. Whether it is still current is not verified.

S1918000013Status unverified
No crank, no start with P0688 (auto shutdown relay sense circuit low)
2019–20221500 · issued November 30, 2020 · NHTSA 10183023
No-crank no-start with P0688 stored in the PCM. The summary does not give the repair.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
S1918000013RevAStatus unverified
No crank, no start with P0688 (Rev. A)
2019–20211500 · issued October 17, 2022 · NHTSA 10223912
Revised version of the bulletin above; same condition.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

All Engine computer (ECM/PCM) codes

P0688 questions

Is it safe to drive with P0688?

It can cause stalling or a no-start, so get it checked soon and stop driving if the engine cuts out while moving.

Do I need a new engine computer?

Rarely at first. Test the battery, relay, fuses and connectors before replacing it.

Sources and review

Guide to P0688 OBD error code — PartsAvatar
Ram bulletin S1918000013: No crank no start with P0688 auto shutdown relay sense circuit low found in the PCM — Ram (via NHTSA manufacturer communications flat file) , November 30, 2020
Ram bulletin S1918000013 Rev.: Crank, no start with P0688 auto shutdown relay sense circuit (starter assembly) — Ram (via NHTSA manufacturer communications flat file) , January 1, 2022
Chrysler bulletin 68232879A: TIPM no-start with P0688 or P0685 active — Chrysler (via NHTSA manufacturer communications flat file) , May 20, 2019

Last reviewed: October 16, 2026
Written by: Saurabh K.
Editorial review: Pending
Technician review: Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

Severity: Inspect soon

Most likely causes

Cause 1: Weak battery or low cranking voltage

Cause 2: Faulty power relay or blown ECM fuse

Cause 3: Loose or corroded relay harness connections

Cause 4: Auto shutdown relay sense fault in the PCM (2019-2022 Ram 1500)

Cause 5: Faulty engine computer or TIPM

What to test first

Step 1: Check battery voltage at rest and while cranking, and clean and tighten the battery cables.

Step 2: With the key off, inspect the engine computer power relay and its fuses, and swap in a known-good relay of the same type if the vehicle allows.

Step 3: Inspect the relay and computer connectors for corrosion or loose terminals, and ask a dealer about a bulletin for P0688 (Ram 1500 2019-2022, Chrysler TIPM).